P25B9 code meaning
Auxiliary Ignition Switch On/Start Position Circuit High
What P25B9 means
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.
P25B9 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P25B9
- Battery drains overnight because modules stay awake
- Auxiliary start function disabled or behaving erratically
- Starter may engage or click unexpectedly
Common causes of P25B9
- Most likelySignal wire chafed against a battery or ignition feed
- PossibleAuxiliary switch stuck in the On/Start position
- PossibleBackfeed through an upfitter relay or accessory
- Less likelyMoisture in an exterior switch box or connector
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Auxiliary ignition switchWiring harness
What each part of P25B9 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| B9 | The exact fault in that area. |
What to do next
- Note P25B9 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P25B9
What does P25B9 mean?
P25B9 means "Auxiliary Ignition Switch On/Start Position Circuit High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P25B9?
P25B9 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P25B9 fail an emissions test?
P25B9 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P25B9 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will P25B9 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.