diff --git a/README.md b/README.md
index 8b773ae..00e62a9 100644
--- a/README.md
+++ b/README.md
@@ -94,6 +94,38 @@ echo "$temp"
 
 Remember to make the file executable (`chmod +x`).
 
+## Fan identity and USB port changes
+
+For USB controllers that expose a unique serial number, fan labels and
+configurations are identified by VID:PID, serial number, interface, and PWM
+channel. The plugin records this identity when a label or configuration is
+saved, and automatically upgrades existing entries at service startup while
+their controller can still be identified. Labels use an encoded `usb:` key in
+`pwm_labels.cfg`; `controller_identity` stores the same key in each fan
+configuration, or a path binding when no unique serial is available. The
+runtime `controller` path is resolved again whenever the service starts.
+
+This preserves assignments when a controller moves to another USB port, even
+when an identical model occupies the old port. Missing or ambiguous identities
+(including duplicate serial numbers for the same channel) remain saved but
+unassigned until they can be resolved; the plugin does not guess another device.
+
+Controllers without a serial number, controllers whose serial is already
+duplicated when first saved, and older entries whose serial has not yet been
+recorded use the same-port path migration. Their existing port assignments
+remain separate. Keep those controllers on the same port. If an unrecorded
+controller has already moved, reassign it once so its serial can be saved.
+Restart the service after a driver reload or port move. If multiple
+configurations resolve to one channel, the already-current assignment wins;
+blocked bindings remain saved for recovery when the conflicting assignment
+is removed.
+Saving a configuration without a resolved controller, including a rename,
+retains its binding. Selecting a different controller replaces the binding.
+Clearing a label also removes its dormant USB identity entry, so it cannot
+reappear after serial ambiguity is resolved.
+The identity identifies the controller's header, not the physical fan attached
+to it; replacing the fan does not change the saved assignment.
+
 Support / Issues
 - https://github.com/andrebrait/fanctrlplus/issues
 
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php b/src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php
index 2e60d7e..9628252 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php
@@ -71,15 +71,6 @@ function normalize_chip_name(string $chip): string {
     return $chip;
 }
 
-function build_pwm_map(): array {
-    $map = [];
-    foreach (list_pwm() as $pwm) {
-        if ($pwm['chip'] === '') continue;
-        $map[normalize_chip_name($pwm['chip']).':'.$pwm['name']] = $pwm['sensor'];
-    }
-    return $map;
-}
-
 function extract_chip_and_pwm_from_path(string $old_path): ?array {
     $old_path = trim($old_path, " \t\n\r\0\x0B\"'");
 
@@ -124,12 +115,92 @@ function extract_chip_and_pwm_from_path(string $old_path): ?array {
     return null;
 }
 
+// Find where a saved /sys/devices pwm path lives now. The hwmonN index follows
+// probe order and a USB HID device gets a new instance suffix
+// (0003:VID:PID.NNNN) each time it enumerates, so both are wildcarded; the
+// rest of the path (bus topology, VID:PID, pwmN) still pins the same device.
+const FCP_HID_INSTANCE = '#(/[0-9A-Fa-f]{4}:[0-9A-Fa-f]{4}:[0-9A-Fa-f]{4})\.[0-9A-Fa-f]+/#';
+
+function find_moved_pwm_path(string $old_path): ?string {
+    // A /sys/class path carries no device identity; glob characters would
+    // turn the saved path into an arbitrary pattern.
+    if (strpos($old_path, '/class/hwmon/') !== false || strpbrk($old_path, '*?[') !== false) return null;
+    if (file_exists($old_path)) return $old_path;
+    $pattern = preg_replace(
+        ['#/hwmon/hwmon\d+/#', FCP_HID_INSTANCE],
+        ['/hwmon/hwmon*/', '$1.*/'],
+        $old_path
+    );
+    $matches = glob($pattern) ?: [];
+    return count($matches) === 1 ? $matches[0] : null;
+}
+
+// Path-only entries cannot recover a serial after the device has moved.
+// Keep the same-port migration for those legacy/no-serial entries.
+function current_pwm_path(string $old_path, array $pwm_map): ?string {
+    $moved = find_moved_pwm_path($old_path);
+    if ($moved !== null || preg_match(FCP_HID_INSTANCE, $old_path) || preg_match('#/usb\d+/#', $old_path)) return $moved;
+    $pair = extract_chip_and_pwm_from_path($old_path);
+    return $pair ? ($pwm_map[$pair[0].':'.$pair[1]] ?? null) : null;
+}
+
+// A fan is a USB controller plus its interface/hwmon channel, not a USB port.
+// Hex-encoded JSON is safe as an unquoted label key and a shell/INI value.
+function fcp_pwm_identity(string $path): ?string {
+    $path = realpath($path);
+    if ($path === false || !preg_match('#/pwm\d+$#', $path)) return null;
+    for ($dir = dirname($path); $dir !== dirname($dir); $dir = dirname($dir)) {
+        if (!is_file("$dir/idVendor") || !is_file("$dir/idProduct")) continue;
+        $vid = strtolower(trim((string)@file_get_contents("$dir/idVendor")));
+        $pid = strtolower(trim((string)@file_get_contents("$dir/idProduct")));
+        $serial = trim((string)@file_get_contents("$dir/serial"));
+        if (!preg_match('/^[0-9a-f]{4}$/', $vid) || !preg_match('/^[0-9a-f]{4}$/', $pid) || $serial === '') return null;
+        $channel = substr($path, strlen($dir));
+        $channel = preg_replace('#^/[^/]+:(\d+\.\d+)/#', '/interface:$1/', $channel);
+        $channel = preg_replace([FCP_HID_INSTANCE, '#/hwmon/hwmon\d+/#'], ['$1.*/', '/hwmon/hwmon*/'], $channel);
+        $encoded = json_encode([$vid, $pid, $serial, $channel]);
+        return $encoded === false ? null : 'usb:'.bin2hex($encoded);
+    }
+    return null;
+}
+
+function fcp_pwm_identity_map(array $pwms): array {
+    $map = [];
+    foreach ($pwms as $pwm) {
+        $key = fcp_pwm_identity($pwm['sensor']);
+        if ($key === null) continue;
+        // Duplicate serials/channels cannot safely identify either device.
+        $map[$key] = array_key_exists($key, $map) ? null : $pwm['sensor'];
+    }
+    return $map;
+}
+
+// Capture a serial key only while it uniquely identifies this channel.
+// Devices with duplicated/placeholder serials retain their port identity.
+function fcp_unique_pwm_identity(string $path, ?array $identities = null): ?string {
+    $key = fcp_pwm_identity($path);
+    if ($key === null) return null;
+    $identities = $identities ?? fcp_pwm_identity_map(list_pwm());
+    return ($identities[$key] ?? null) === realpath($path) ? $key : null;
+}
+
+function fcp_load_pwm_labels(string $file, string $hwmon_glob = '/sys/class/hwmon/hwmon*'): array {
+    $identities = fcp_pwm_identity_map(list_pwm($hwmon_glob));
+    $labels = [];
+    foreach (is_file($file) ? file($file, FILE_IGNORE_NEW_LINES) : [] as $line) {
+        if (!preg_match('/^(.+?)=(.+)$/', $line, $m)) continue;
+        $path = strpos($m[1], 'usb:') === 0 ? ($identities[$m[1]] ?? null) : $m[1];
+        if ($path !== null && preg_match('#/pwm\d+$#', $path)) $labels[$path] = $m[2];
+    }
+    return $labels;
+}
+
 function log_migrate(string $msg): void {
     // Write the plugin log.
     @file_put_contents("/var/log/fanctrlplus2-migrate.log",
         date("c")." ".$msg."\n", FILE_APPEND);
     // Mirror the entry to syslog.
-    @exec("logger -t fanctrlplus2 '$msg'");
+    @exec('logger -t fanctrlplus2 ' . escapeshellarg($msg));
 }
 
 function safe_rewrite(string $file, string $content): bool {
@@ -142,81 +213,117 @@ function safe_rewrite(string $file, string $content): bool {
     return true;
 }
 
-function migrate_cfg_and_labels(string $plugin): void {
-    $cfgpath   = "/boot/config/plugins/$plugin";
+function migrate_cfg_and_labels(string $plugin, string $cfgpath = '', string $hwmon_glob = '/sys/class/hwmon/hwmon*'): void {
+    $cfgpath   = $cfgpath ?: "/boot/config/plugins/$plugin";
     $labelFile = "$cfgpath/pwm_labels.cfg";
-    $pwm_map   = build_pwm_map();
+    $pwms = list_pwm($hwmon_glob);
+    $pwm_map = [];
+    foreach ($pwms as $pwm) $pwm_map[normalize_chip_name($pwm['chip']).':'.$pwm['name']] = $pwm['sensor'];
+    $identities = fcp_pwm_identity_map($pwms);
 
     // --- labels ---
     if (is_file($labelFile)) {
         $lines = file($labelFile, FILE_IGNORE_NEW_LINES) ?: [];
-        $changed = false; $out = [];
+        // Several lines can land on one path: a label re-entered for the
+        // current path after the device moved, or stale ones from several
+        // moves. Keep one line, as the readers see it (the last one), except
+        // that a line saved for the current path beats any migrated one.
+        $changed = false; $out = []; $pos = []; $current = [];
         foreach ($lines as $line) {
             if (!preg_match('/^(.+?)=(.*)$/', $line, $m)) { $out[]=$line; continue; }
             $old_path = trim($m[1], " \t\n\r\0\x0B\"'");
             $label    = $m[2];
 
-            if (preg_match('/^__FCP_[A-Z0-9_]+__$/', $old_path)) {
+            // Flags, and empty labels, which the readers ignore.
+            if (preg_match('/^__FCP_[A-Z0-9_]+__$/', $old_path) || $label === '') {
                 $out[] = $line;
                 continue;
             }
 
-            $pair = extract_chip_and_pwm_from_path($old_path);
-            if (!$pair) { log_migrate("migrate label: skip (unparsable) $old_path"); $out[]=$line; continue; }
-            [$chip,$pwmN] = $pair;
-            $key = "$chip:$pwmN";
-            if (!isset($pwm_map[$key])) { log_migrate("migrate label: no match for $chip:$pwmN, keep $old_path"); $out[]=$line; continue; }
-
-            $new_path = $pwm_map[$key];
-            if ($new_path !== $old_path) {
+            $stable = strpos($old_path, 'usb:') === 0;
+            $new_path = $stable ? ($identities[$old_path] ?? null) : current_pwm_path($old_path, $pwm_map);
+            if ($new_path === null) { log_migrate("migrate label: no match, keep $old_path"); $out[]=$line; continue; }
+            $is_current = $stable || $new_path === $old_path;
+            $key = fcp_unique_pwm_identity($new_path, $identities) ?? $new_path;
+            if ($key !== $old_path) $changed = true;
+            if (!$is_current) {
+                $changed = true;
                 if (preg_match('#/(hwmon\d+)/#', $old_path, $o) && preg_match('#/(hwmon\d+)/#', $new_path, $n)) {
                     log_migrate("migrate label: $old_path → $new_path ({$o[1]} → {$n[1]})");
                 } else {
                     log_migrate("migrate label: $old_path → $new_path");
                 }
-                $changed = true;
-                $out[] = $new_path.'='.$label;
+            }
+            if (!isset($pos[$key])) {
+                $pos[$key] = count($out);
+                $out[] = $key.'='.$label;
             } else {
-                $out[] = $line;
+                $changed = true;
+                if (!$is_current && isset($current[$key])) {
+                    log_migrate("migrate label: drop $old_path, $new_path already labeled");
+                    continue;
+                }
+                $out[$pos[$key]] = $key.'='.$label;
             }
+            if ($is_current) $current[$key] = true;
         }
         if ($changed) safe_rewrite($labelFile, implode("\n", $out));
     }
 
     // --- cfgs ---
-    foreach (glob("$cfgpath/{$plugin}_*.cfg") ?: [] as $cfgfile) {
+    // A fan assigned to the current path while an old one still pointed at
+    // the vanished path keeps it: two loops must not drive the same PWM.
+    $cfgfiles = glob("$cfgpath/{$plugin}_*.cfg") ?: [];
+    $configs = []; $in_use = [];
+    foreach ($cfgfiles as $cfgfile) {
         $ini = @parse_ini_file($cfgfile);
         if (!$ini) continue;
-
+        $configs[$cfgfile] = $ini;
+        $path = (string)($ini['controller'] ?? '');
+        $identity = (string)($ini['controller_identity'] ?? '');
+        $source = $identity !== '' ? $identity : $path;
+        $resolved = strpos($source, 'usb:') === 0 ? ($identities[$source] ?? null) : (is_file($source) ? $source : null);
+        // Already-current manual assignments win over stale migrated ones.
+        if ($resolved === $path && $path !== '' && !isset($in_use[$path])) $in_use[$path] = $cfgfile;
+    }
+    foreach ($configs as $cfgfile => $ini) {
+        $original = $ini;
         $old_path = trim((string)($ini['controller'] ?? ''), " \t\n\r\0\x0B\"'");
-
-        if ($old_path === '' || !preg_match('#/hwmon\d+/pwm\d+$#', $old_path)) {
-            continue;
-        }
-
-        $pair = extract_chip_and_pwm_from_path($old_path);
-        if (!$pair) { 
-            log_migrate("migrate cfg: skip (unparsable) $cfgfile controller=$old_path"); 
-            continue; 
+        $identity = (string)($ini['controller_identity'] ?? '');
+        if ($identity === '' && !preg_match('#/pwm\d+$#', $old_path)) continue;
+        $source = $identity !== '' ? $identity : $old_path;
+        $new_path = strpos($source, 'usb:') === 0 ? ($identities[$source] ?? null) : current_pwm_path($source, $pwm_map);
+        if ($new_path !== null && isset($in_use[$new_path]) && $in_use[$new_path] !== $cfgfile) {
+            log_migrate("migrate cfg: keep $old_path in $cfgfile, $new_path is already controlled");
+            if ($identity === '' && !is_file($old_path)) continue;
+            // A live losing path must also stop. Retain its binding for
+            // recovery when the winning assignment is removed.
+            if ($identity === '') $identity = fcp_unique_pwm_identity($old_path, $identities) ?? $old_path;
+            $ini['controller_identity'] = $identity;
+            $new_path = null;
         }
-        [$chip,$pwmN] = $pair;
-        $key = "$chip:$pwmN";
-        if (!isset($pwm_map[$key])) { 
-            log_migrate("migrate cfg: no match for $cfgfile ($chip:$pwmN), keep $old_path"); 
-            continue; 
-        }
-
-        $new_path = $pwm_map[$key];
-        if ($new_path === $old_path) continue;
-
-        if (preg_match('#/(hwmon\d+)/#', $old_path, $o) && preg_match('#/(hwmon\d+)/#', $new_path, $n)) {
-            log_migrate("migrate cfg: $cfgfile controller: $old_path → $new_path ({$o[1]} → {$n[1]})");
+        if ($new_path === null) {
+            log_migrate("migrate cfg: no match for $cfgfile, identity=$identity");
+            // Never drive a replacement at the old path. Keep the identity
+            // so a disconnected/ambiguous controller can recover later.
+            if ($identity === '') continue;
+            $ini['controller'] = '';
         } else {
-            log_migrate("migrate cfg: $cfgfile controller: $old_path → $new_path");
+            $in_use[$new_path] = $cfgfile;
+            $ini['controller'] = $new_path;
+            $ini['controller_identity'] = fcp_unique_pwm_identity($new_path, $identities) ?? $new_path;
+            if ($new_path !== $old_path) log_migrate("migrate cfg: $cfgfile controller: $old_path → $new_path");
+        }
+        if ($ini === $original) continue;
+        // Only these two fields changed. Re-serializing the entire INI would
+        // re-escape free-text values that are also sourced by the shell.
+        $buf = (string)file_get_contents($cfgfile);
+        foreach (['controller', 'controller_identity'] as $k) {
+            $v = str_replace(["\\", '"', '$', '`', "\r", "\n"], ["\\\\", '\\"', '\\$', '\\`', '', ''], (string)$ini[$k]);
+            $line = "$k=\"$v\"";
+            $buf = preg_replace_callback('/^[ \t]*'.$k.'[ \t]*=.*$/m', function () use ($line) { return $line; }, $buf, -1, $count);
+            if ($count === 0) $buf = rtrim($buf, "\n")."\n".$line."\n";
         }
-
-        $ini['controller'] = $new_path;
-        $buf=''; foreach ($ini as $k=>$v){ $v=str_replace('"','',(string)$v); $buf.=$k.'="'.$v."\"\n"; }
         safe_rewrite($cfgfile, $buf);
     }
 }
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanBlockRender.php b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanBlockRender.php
index 61755cf..b7d7ef1 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanBlockRender.php
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanBlockRender.php
@@ -1,13 +1,7 @@
 <?php
+require_once __DIR__.'/Common.php';
 $label_file = "/boot/config/plugins/fanctrlplus2/pwm_labels.cfg";
-$pwm_labels = [];
-if (is_file($label_file)) {
-  foreach (file($label_file, FILE_IGNORE_NEW_LINES) as $line) {
-    if (preg_match('/^(.+?)=(.+)$/', $line, $m)) {
-      $pwm_labels[$m[1]] = $m[2];
-    }
-  }
-}
+$pwm_labels = fcp_load_pwm_labels($label_file);
 
 // Build the list of disk groups for a fan cfg: [{name, disks:[...], low, high}, ...].
 // disk_group_count>0 -> read disk_group_{g}_* keys. Otherwise fall back to ONE group
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanctrlLogic.php b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanctrlLogic.php
index 4b85c3c..0363fce 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanctrlLogic.php
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FanctrlLogic.php
@@ -110,38 +110,21 @@ function scan_dir($dir) {
     $pwm = $_POST['pwm'] ?? '';
     $label = $_POST['label'] ?? '';
 
-    $label_file = "/boot/config/plugins/fanctrlplus2/pwm_labels.cfg";
-    // Read existing labels.
-    $lines = is_file($label_file) ? file($label_file, FILE_IGNORE_NEW_LINES) : [];
-    $found = false;
-
     if (!$pwm) {
       json_response(['status' => 'error', 'message' => 'Missing pwm']);
       break;
     }
-
-    // An empty label means delete it.
-    if ($label === '') {
-      $new_lines = [];
-      foreach ($lines as $line) {
-        if (strpos($line, "$pwm=") !== 0) $new_lines[] = $line;
-      }
-      file_put_contents($label_file, implode("\n", $new_lines) . "\n");
-      json_response(['status' => 'ok', 'message' => 'Label removed']);
-      break;
-    }
-
-    // Write the label.
-    foreach ($lines as &$line) {
-      if (strpos($line, "$pwm=") === 0) {
-        $line = "$pwm=$label";
-        $found = true;
-        break;
-      }
-    }
-    if (!$found) $lines[] = "$pwm=$label";
+    $identity = fcp_pwm_identity($pwm);
+    $key = fcp_unique_pwm_identity($pwm) ?? $pwm;
+    $lines = is_file($label_file) ? file($label_file, FILE_IGNORE_NEW_LINES) : [];
+    // Remove both the legacy path and stable key before saving one entry.
+    $lines = array_values(array_filter($lines, function ($line) use ($key, $pwm, $identity, $label) {
+      return strpos($line, "$key=") !== 0 && strpos($line, "$pwm=") !== 0
+        && ($label !== '' || $identity === null || strpos($line, "$identity=") !== 0);
+    }));
+    if ($label !== '') $lines[] = "$key=$label";
     file_put_contents($label_file, implode("\n", $lines) . "\n");
-    json_response(['status' => 'ok', 'message' => 'Label saved']);
+    json_response(['status' => 'ok', 'message' => $label === '' ? 'Label removed' : 'Label saved']);
     break;
   
   case 'newtemp':
@@ -324,14 +307,7 @@ function scan_dir($dir) {
   case 'getpwm':
     $pwms = list_pwm();
     $label_file = "/boot/config/plugins/fanctrlplus2/pwm_labels.cfg";
-    $labels = [];
-    if (is_file($label_file)) {
-      foreach (file($label_file, FILE_IGNORE_NEW_LINES) as $line) {
-        if (preg_match('/^(.+?)=(.+)$/', $line, $m)) {
-          $labels[$m[1]] = $m[2];
-        }
-      }
-    }
+    $labels = fcp_load_pwm_labels($label_file);
     foreach ($pwms as &$pwm) {
       $pwm['label'] = $labels[$pwm['sensor']] ?? '';
     }
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FcpAirflow.php b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FcpAirflow.php
index 003e82e..f98fa97 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/include/FcpAirflow.php
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/include/FcpAirflow.php
@@ -1,27 +1,15 @@
 <?php
 header('Content-Type: application/json; charset=utf-8');
 header('Cache-Control: no-store, max-age=0');
+require_once __DIR__.'/Common.php';
 
 /**
- * Read /abs/.../pwmN=Name entries from pwm_labels.cfg.
- * Use realpath(dirname(...)) plus channel N as the key and map it to fanN_input.
+ * Resolve saved labels to current PWM paths, then map them to fanN_input.
  */
 function load_labels(): array {
   $cfg = '/boot/config/plugins/fanctrlplus2/pwm_labels.cfg';
   $dirN_to_label = []; // key = realdir.'::'.N => label
-  if (!is_file($cfg)) return $dirN_to_label;
-
-  $lines = file($cfg, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
-  foreach ($lines as $line) {
-    $line = trim($line);
-    if ($line === '' || $line[0] === '#') continue;
-
-    $eq = strpos($line, '=');
-    if ($eq === false) continue;
-
-    $pwm_path = trim(substr($line, 0, $eq));
-    $label    = trim(substr($line, $eq+1));
-    if ($label === '') continue;
+  foreach (fcp_load_pwm_labels($cfg) as $pwm_path => $label) {
 
     if (preg_match('~/pwm(\d+)$~i', $pwm_path, $m)) {
       $n    = (int)$m[1];
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/include/update.fanctrlplus2.php b/src/usr/local/emhttp/plugins/fanctrlplus2/include/update.fanctrlplus2.php
index 76bbd84..d98dc95 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/include/update.fanctrlplus2.php
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/include/update.fanctrlplus2.php
@@ -47,6 +47,12 @@
   $expected_file = $plugin . '_' . $custom . '.cfg';
   $old_path = "$cfgpath/$old_file";
   $new_path = "$cfgpath/$expected_file";
+  // Read before rename/overwrite: an empty runtime controller means the
+  // saved binding is unresolved, not that the user selected another device.
+  $previous = is_file($old_path) ? (@parse_ini_file($old_path) ?: []) : [];
+  $controller_identity = $controller === ''
+    ? (string)($previous['controller_identity'] ?? $previous['controller'] ?? '')
+    : (fcp_unique_pwm_identity($controller) ?? $controller);
   
   // Read the raw values first.
   $pwm_percent_raw = $_POST['pwm_percent'][$i] ?? '';
@@ -247,6 +253,7 @@
     'label'      => $custom,
     'service'    => $_POST['service'][$i] ?? '0',
     'controller' => $controller,
+    'controller_identity' => $controller_identity,
     'pwm'        => $pwm,
     'max'        => $max_pwm,
     'idle'       => (string)$idle_abs,
diff --git a/src/usr/local/emhttp/plugins/fanctrlplus2/scripts/rc.fanctrlplus2 b/src/usr/local/emhttp/plugins/fanctrlplus2/scripts/rc.fanctrlplus2
index 434b0e1..0541630 100644
--- a/src/usr/local/emhttp/plugins/fanctrlplus2/scripts/rc.fanctrlplus2
+++ b/src/usr/local/emhttp/plugins/fanctrlplus2/scripts/rc.fanctrlplus2
@@ -36,6 +36,10 @@ fanctrlplus2.start() {
     service=$(grep -Po '^service="\K[^"]+' "$cfg")
     [[ "$service" != "1" ]] && continue
 
+    # Serial resolution can leave an absent/ambiguous controller unassigned.
+    controller=$(grep -Po '^controller="\K[^"]+' "$cfg")
+    [[ -n "$controller" && -w "$controller" ]] || continue
+
     # Sanitize the custom name before using it in a PID filename.
     raw_custom=$(grep -Po '^custom="\K[^"]+' "$cfg")
     clean_custom=$(echo "$raw_custom" | tr -d '\r\n\t ')
diff --git a/tests/pwm_path_migration_test.php b/tests/pwm_path_migration_test.php
new file mode 100644
index 0000000..b55d477
--- /dev/null
+++ b/tests/pwm_path_migration_test.php
@@ -0,0 +1,99 @@
+<?php
+
+require_once __DIR__ . '/../src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php';
+
+$failures = [];
+$root = sys_get_temp_dir() . '/fcp_pwm_migration_' . getmypid();
+$port = "$root/devices/pci0000:00/0000:00:14.0/usb1/1-14/1-14:1.0";
+
+$make = function (string $path): void {
+    @mkdir(dirname($path), 0777, true);
+    touch($path);
+};
+$make("$root/devices/platform/nct6775.656/hwmon/hwmon2/pwm2");
+$make("$port/0003:3904:F001.0008/hwmon/hwmon7/pwm9");
+$make("$root/devices/pci0000:00/0000:00:14.0/usb1/1-9/1-9:1.0/0003:3904:F001.0009/hwmon/hwmon8/pwm9");
+
+// The issue's case: the HID instance and the hwmon index both changed.
+$found = find_moved_pwm_path("$port/0003:3904:F001.0005/hwmon/hwmon5/pwm9");
+if ($found !== "$port/0003:3904:F001.0008/hwmon/hwmon7/pwm9") {
+    $failures[] = 'A re-enumerated USB fan controller must be found on its own port, got ' . var_export($found, true);
+}
+
+$found = find_moved_pwm_path("$root/devices/platform/nct6775.656/hwmon/hwmon4/pwm2");
+if ($found !== "$root/devices/platform/nct6775.656/hwmon/hwmon2/pwm2") {
+    $failures[] = 'A renumbered hwmon index must resolve to the same platform device, got ' . var_export($found, true);
+}
+
+// An identical hub on another port is a different device.
+if (find_moved_pwm_path("$root/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3:1.0/0003:3904:F001.0005/hwmon/hwmon5/pwm9") !== null) {
+    $failures[] = 'A controller must not be matched to an identical one on a different USB port.';
+}
+if (find_moved_pwm_path("$port/0003:3904:F001.0005/hwmon/hwmon5/pwm3") !== null) {
+    $failures[] = 'A channel the device no longer has must not match.';
+}
+// Two instances of the same device on one port: nothing pins either one.
+$twin = "$root/devices/pci0000:00/0000:00:14.0/usb2/2-1/2-1:1.0";
+$make("$twin/0003:3904:F001.000A/hwmon/hwmon9/pwm1");
+$make("$twin/0003:3904:F001.000B/hwmon/hwmon10/pwm1");
+if (find_moved_pwm_path("$twin/0003:3904:F001.0002/hwmon/hwmon3/pwm1") !== null) {
+    $failures[] = 'An ambiguous match must not be picked.';
+}
+if (find_moved_pwm_path("$twin/0003:3904:F001.000A/hwmon/hwmon9/pwm1") !== "$twin/0003:3904:F001.000A/hwmon/hwmon9/pwm1") {
+    $failures[] = 'A path that still exists must be kept even when it has siblings.';
+}
+
+$make("$root/class/hwmon/hwmon3/pwm2");
+if (find_moved_pwm_path("$root/class/hwmon/hwmon5/pwm2") !== null) {
+    $failures[] = 'A /sys/class path carries no device identity and must be left to the chip-name fallback.';
+}
+if (find_moved_pwm_path("$root/devices/platform/nct6775.*/hwmon/hwmon4/pwm2") !== null) {
+    $failures[] = 'A saved path must not be used as a glob pattern.';
+}
+
+// The reporter's state after renumbering twice and re-assigning by hand:
+// stale lines for older instances next to new ones for the current path.
+$make("$port/0003:3904:F001.0008/hwmon/hwmon7/pwm8");
+$make("$port/0003:3904:F001.0008/hwmon/hwmon7/pwm7");
+$older = "$port/0003:3904:F001.0003/hwmon/hwmon4";
+$old = "$port/0003:3904:F001.0005/hwmon/hwmon5";
+$new = "$port/0003:3904:F001.0008/hwmon/hwmon7";
+$cfg = "$root/cfg";
+@mkdir($cfg);
+file_put_contents("$cfg/pwm_labels.cfg", implode("\n", [
+    '__FCP_HISTORY__=1',
+    "$old/pwm9=JBOD_old",
+    "$old/pwm8=Rear",
+    "$new/pwm8=",
+    "$new/pwm9=JBOD",
+    "$older/pwm9=JBOD_older",
+    "$older/pwm7=Top_older",
+    "$old/pwm7=Top",
+]) . "\n");
+file_put_contents("$cfg/fanctrlplus2_old.cfg", "custom=\"old\"\ncontroller=\"$old/pwm9\"\n");
+file_put_contents("$cfg/fanctrlplus2_new.cfg", "custom=\"new\"\ncontroller=\"$new/pwm9\"\n");
+file_put_contents("$cfg/fanctrlplus2_rear.cfg", "custom=\"rear\"\ncontroller=\"$old/pwm8\"\n");
+file_put_contents("$cfg/fanctrlplus2_rear2.cfg", "custom=\"rear2\"\ncontroller=\"$older/pwm8\"\n");
+
+migrate_cfg_and_labels('fanctrlplus2', $cfg);
+
+$labels = file("$cfg/pwm_labels.cfg", FILE_IGNORE_NEW_LINES);
+if ($labels !== ['__FCP_HISTORY__=1', "$new/pwm9=JBOD", "$new/pwm8=Rear", "$new/pwm8=", "$new/pwm7=Top"]) {
+    $failures[] = "Each current path must keep one label: the one saved for it, else the last stale one. Got:\n" . implode("\n", $labels);
+}
+$controller = fn(string $name) => parse_ini_file("$cfg/fanctrlplus2_$name.cfg")['controller'];
+if ($controller('old') !== "$old/pwm9") {
+    $failures[] = 'A fan must not be migrated onto a PWM another fan already controls.';
+}
+if ($controller('rear') !== "$new/pwm8" || $controller('rear2') !== "$older/pwm8") {
+    $failures[] = 'Exactly one of two stale fans on the same PWM must be migrated to its current path.';
+}
+
+exec('rm -rf ' . escapeshellarg($root));
+
+if ($failures) {
+    fwrite(STDERR, implode("\n", $failures) . "\n");
+    exit(1);
+}
+
+echo "pwm path migration tests passed\n";
diff --git a/tests/usb_identity_test.php b/tests/usb_identity_test.php
new file mode 100644
index 0000000..333a5fe
--- /dev/null
+++ b/tests/usb_identity_test.php
@@ -0,0 +1,143 @@
+<?php
+require_once __DIR__.'/../src/usr/local/emhttp/plugins/fanctrlplus2/include/Common.php';
+
+$root = realpath(sys_get_temp_dir()).'/fcp_usb_identity_'.getmypid();
+$cfg = "$root/cfg";
+$hwmon = "$root/class/hwmon";
+mkdir($cfg, 0777, true);
+mkdir($hwmon, 0777, true);
+$failures = [];
+$check = function ($condition, string $message) use (&$failures) {
+    if (!$condition) $failures[] = $message;
+};
+$device = function (string $port, string $index, string $serial, string $instance = '0008') use ($root, $hwmon): string {
+    $usb = "$root/devices/$port";
+    $dir = "$usb/$port:1.0/0003:3904:F001.$instance/hwmon/$index";
+    mkdir($dir, 0777, true);
+    file_put_contents("$usb/idVendor", "3904\n");
+    file_put_contents("$usb/idProduct", "f001\n");
+    file_put_contents("$usb/serial", "$serial\n");
+    file_put_contents("$dir/name", "arctic_fan\n");
+    touch("$dir/pwm10");
+    symlink($dir, "$hwmon/$index");
+    return "$dir/pwm10";
+};
+$path = $device('1-14', 'hwmon5', '2080376E4548');
+$file = "$cfg/fanctrlplus2_jbod.cfg";
+$metadata = 'disk_group_0_name="Top \` (group)"'."\n";
+file_put_contents($file, "custom=\"jbod\"\nservice=\"1\"\ncontroller=\"$path\"\n".$metadata);
+file_put_contents("$cfg/pwm_labels.cfg", "$path=JBOD\n__FCP_HISTORY__=1\n");
+$migrate = function () use ($cfg, $hwmon) { migrate_cfg_and_labels('fanctrlplus2', $cfg, "$hwmon/hwmon*"); };
+$migrate();
+$identity = parse_ini_file($file)['controller_identity'] ?? '';
+$check($identity !== '', 'Startup must persist USB serial identity before a port move.');
+$check(strpos(file_get_contents($file), $metadata) !== false, 'Identity capture must not change unrelated escaped group labels.');
+
+if ($identity !== '') {
+    // Move the controller, then install an identical model at its old port.
+    rename("$root/devices/1-14", "$root/removed");
+    unlink("$hwmon/hwmon5");
+    $moved = $device('2-3.4.5', 'hwmon9', '2080376E4548', '0011');
+    $replacement = $device('1-14', 'hwmon5', 'DIFFERENT');
+    $migrate();
+    $check(parse_ini_file($file)['controller'] === $moved, 'Serial must follow a move across buses/hubs instead of the replacement at the old port.');
+    $check(fcp_load_pwm_labels("$cfg/pwm_labels.cfg", "$hwmon/hwmon*") === [$moved => 'JBOD'], 'Labels must follow the serial and keep channel 10.');
+    $check(parse_ini_file($file)['controller_identity'] === $identity, 'Re-enumeration must not change the saved serial/channel identity.');
+    $check(strpos(file_get_contents($file), $metadata) !== false, 'A port move must preserve unrelated configuration bytes.');
+    $check(strpos(file_get_contents("$cfg/pwm_labels.cfg"), '__FCP_HISTORY__=1') !== false, 'Serial migration must preserve dashboard flags.');
+
+    // Duplicate serial numbers cannot safely identify either controller.
+    $duplicate = $device('3-2', 'hwmon12', '2080376E4548');
+    $migrate();
+    $check(parse_ini_file($file)['controller'] === '', 'Duplicate serials must disable the runtime controller, not guess.');
+    $check(parse_ini_file($file)['controller_identity'] === $identity, 'Unresolved identity must remain saved for recovery.');
+    $check(fcp_load_pwm_labels("$cfg/pwm_labels.cfg", "$hwmon/hwmon*") === [], 'Ambiguous serial labels must not attach to either device.');
+
+    unlink("$hwmon/hwmon12");
+    $migrate();
+    $check(parse_ini_file($file)['controller'] === $moved, 'A resolved serial must restore its runtime path after ambiguity clears.');
+    $check(fcp_load_pwm_labels("$cfg/pwm_labels.cfg", "$hwmon/hwmon*") === [$moved => 'JBOD'], 'Labels must survive ambiguity and reappear on recovery.');
+    unlink("$hwmon/hwmon9");
+    $migrate();
+    $check(parse_ini_file($file)['controller'] === '', 'An absent serial must not fall back to an identical replacement.');
+
+    // No serial: keep the legacy same-port behavior, not a VID:PID guess.
+    $noSerial = $device('4-1', 'hwmon15', '');
+    $check(fcp_pwm_identity($noSerial) === null, 'A device without a serial must not get a serial identity.');
+    $legacyFile = "$cfg/fanctrlplus2_legacy.cfg";
+    file_put_contents($legacyFile, "custom=\"legacy\"\ncontroller=\"$noSerial\"\n");
+    $migrate();
+    $check(strpos((string)(parse_ini_file($legacyFile)['controller_identity'] ?? ''), 'usb:') !== 0, 'A serial-less configuration must retain its path identity.');
+    rename("$root/devices/4-1", "$root/removed_no_serial");
+    unlink("$hwmon/hwmon15");
+    $noSerialMoved = $device('4-2', 'hwmon16', '');
+    $migrate();
+    $check(parse_ini_file($legacyFile)['controller'] !== $noSerialMoved, 'A serial-less port move must not be guessed by VID:PID.');
+
+    // Duplicate serials present at upgrade must not collapse working port
+    // assignments into an ambiguous key.
+    $a = $device('5-1', 'hwmon20', 'FAKE');
+    $b = $device('5-2', 'hwmon21', 'FAKE');
+    $dupCfg = "$root/duplicates";
+    mkdir($dupCfg);
+    file_put_contents("$dupCfg/pwm_labels.cfg", "$a=Front\n$b=Rear\n");
+    file_put_contents("$dupCfg/fanctrlplus2_a.cfg", "controller=\"$a\"\n");
+    file_put_contents("$dupCfg/fanctrlplus2_b.cfg", "controller=\"$b\"\n");
+    for ($i = 0; $i < 2; $i++) migrate_cfg_and_labels('fanctrlplus2', $dupCfg, "$hwmon/hwmon*");
+    $check(fcp_load_pwm_labels("$dupCfg/pwm_labels.cfg", "$hwmon/hwmon*") === [$a=>'Front', $b=>'Rear'], 'Ambiguous serials must leave both existing path labels intact.');
+    $check(parse_ini_file("$dupCfg/fanctrlplus2_a.cfg")['controller'] === $a && parse_ini_file("$dupCfg/fanctrlplus2_b.cfg")['controller'] === $b, 'Ambiguous serials at capture must not disable existing port assignments.');
+
+    // A manually reassigned current path wins over a moved identity,
+    // independent of the two filenames.
+    foreach ([['aa', 'zz'], ['zz', 'aa']] as [$serialName, $liveName]) {
+        $collision = "$root/collision_$serialName";
+        mkdir($collision);
+        $serialFile = "$collision/fanctrlplus2_$serialName.cfg";
+        $liveFile = "$collision/fanctrlplus2_$liveName.cfg";
+        $known = fcp_pwm_identity($replacement);
+        file_put_contents($serialFile, "controller=\"$root/gone/hwmon1/pwm10\"\ncontroller_identity=\"$known\"\n");
+        file_put_contents($liveFile, "controller=\"$replacement\"\n");
+        migrate_cfg_and_labels('fanctrlplus2', $collision, "$hwmon/hwmon*");
+        $check(parse_ini_file($liveFile)['controller'] === $replacement && parse_ini_file($serialFile)['controller'] === '', 'A current manual assignment must win without two loops, in either filename order.');
+        unlink($liveFile);
+        migrate_cfg_and_labels('fanctrlplus2', $collision, "$hwmon/hwmon*");
+        $check(parse_ini_file($serialFile)['controller'] === $replacement, 'A blocked identity must recover when the other assignment is removed.');
+    }
+
+    // Two path-only configs on one live channel also cannot launch two
+    // loops. The losing path binding remains available for later recovery.
+    $same = "$root/same_channel";
+    mkdir($same);
+    $first = "$same/fanctrlplus2_a.cfg";
+    $second = "$same/fanctrlplus2_b.cfg";
+    file_put_contents($first, "controller=\"$noSerialMoved\"\n");
+    file_put_contents($second, "controller=\"$noSerialMoved\"\n");
+    migrate_cfg_and_labels('fanctrlplus2', $same, "$hwmon/hwmon*");
+    $check(parse_ini_file($first)['controller'] === $noSerialMoved && parse_ini_file($second)['controller'] === '', 'Two live path-only assignments must leave exactly one runtime controller.');
+    unlink($first);
+    migrate_cfg_and_labels('fanctrlplus2', $same, "$hwmon/hwmon*");
+    $check(parse_ini_file($second)['controller'] === $noSerialMoved, 'A blocked path binding must recover when its owner is removed.');
+
+    // USB drivers need not be HID drivers: the relative path does not
+    // itself carry VID:PID, so the identity must include both explicitly.
+    $nonHid = [];
+    foreach ([['6-1', 'hwmon30', '1111', '2222'], ['6-2', 'hwmon31', '3333', '4444']] as [$port, $index, $vid, $pid]) {
+        $usb = "$root/devices/$port";
+        $dir = "$usb/$port:1.0/hwmon/$index";
+        mkdir($dir, 0777, true);
+        foreach (['idVendor'=>$vid, 'idProduct'=>$pid, 'serial'=>'SHARED'] as $key=>$value) file_put_contents("$usb/$key", "$value\n");
+        file_put_contents("$dir/name", "usb_fan\n");
+        touch("$dir/pwm10");
+        symlink($dir, "$hwmon/$index");
+        $nonHid[] = "$dir/pwm10";
+    }
+    $keyA = fcp_pwm_identity($nonHid[0]);
+    $keyB = fcp_pwm_identity($nonHid[1]);
+    $check($keyA !== null && $keyB !== null && $keyA !== $keyB, 'VID:PID must distinguish devices reporting the same serial.');
+    $map = fcp_pwm_identity_map(list_pwm("$hwmon/hwmon*"));
+    $check(($map[$keyA] ?? null) === $nonHid[0] && ($map[$keyB] ?? null) === $nonHid[1], 'Different VID:PID pairs must not become an ambiguous serial.');
+}
+
+exec('rm -rf '.escapeshellarg($root));
+if ($failures) { fwrite(STDERR, implode("\n", $failures)."\n"); exit(1); }
+echo "USB serial identity tests passed\n";