P25B9Auxiliary Ignition Switch On/Start Position Circuit High
- Generic OBD-II code · Powertrain
- How the car detects it
- With P25B9 the module sees the auxiliary On/Start line at a high level when it ought to be inactive, for example after key-off or with no request from the remote panel. A switch stuck in the start position, a wire rubbing against a battery feed, backfeed through an upfitter relay or moisture bridging terminals in an exterior switch box can all hold the line up. The consequences differ from the low fault: a permanent start request can keep control modules awake and flatten the battery, and in the worst case could engage the starter unexpectedly. On its own wake-up input, GM's 2025 brake module (25OBDG04A) logs P2535 when the line stays high for 3 s although the ECM reports it should be low. P25B9 itself is not described publicly by any manufacturer, so diagnosis follows the upfitter wiring diagram.
- What you may notice
- Battery drains overnight because modules stay awake
- Auxiliary start function disabled or behaving erratically
- Starter may engage or click unexpectedly
- Likely causes
- LikelySignal wire chafed against a battery or ignition feed
- PossibleAuxiliary switch stuck in the On/Start position
- PossibleBackfeed through an upfitter relay or accessory
- RareMoisture in an exterior switch box or connector
- Repair
- Medium · Usually a job for a workshop.Typical cost: €60 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.