P25B8 code meaning
Auxiliary Ignition Switch On/Start Position Circuit Low
What P25B8 means
P25B8 means the auxiliary On/Start input sits at a low level when the module expects it to be active, or never rises when the remote switch is operated. Typical reasons are a switch contact that no longer closes, a blown fuse feeding the remote panel, a broken wire in the long harness run to the body or a short to ground at a frame connector. In practice it behaves like a dead remote switch: requests from the auxiliary position are ignored, so a crew working at the body or platform cannot start the engine or keep it on from there. As no maker publishes this code, the GM main-switch case serves as reference, where GM's brake control module needs its wake-up line low for 3 s against a run/crank message from the ECM before logging P2534 as a Type B fault. P25B9 covers the opposite direction.
P25B8 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 P25B8
- Remote start or on request is ignored
- Engine may shut down when the auxiliary position is used to keep it running
- Remote panel indicator stays dark
Common causes of P25B8
- Most likelyBlown fuse or failed relay feeding the auxiliary switch
- PossibleSignal wire shorted to ground on the frame or body
- PossibleAuxiliary switch contact no longer closes
- Less likelyCorroded exterior connector with high resistance
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 P25B8 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. |
| B8 | The exact fault in that area. |
What to do next
- Note P25B8 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 P25B8
What does P25B8 mean?
P25B8 means "Auxiliary Ignition Switch On/Start Position Circuit Low". P25B8 means the auxiliary On/Start input sits at a low level when the module expects it to be active, or never rises when the remote switch is operated. It is a generic code, so it means the same on every make.
Is it safe to drive with P25B8?
P25B8 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P25B8 fail an emissions test?
P25B8 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 P25B8 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will P25B8 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.