Powertrain, generic (extended)

P25B8 Auxiliary Ignition Switch On/Start Position Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Blown fuse or failed relay feeding the auxiliary switch
Signal wire shorted to ground on the frame or body
Auxiliary switch contact no longer closes
Corroded exterior connector with high resistance

Symptoms

  • 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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–400
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

auxiliary ignition switchwiring harness

Related codes

P25B7P25B9P2534

References: gsi.ext.gm.com, http:, law.cornell.edu.