P25B9 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. 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.

Likely causes

Symptoms

• Battery drains overnight because modules stay awake
• Auxiliary start function disabled or behaving erratically
• Starter may engage or click unexpectedly

Repair estimate

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

Parts involved

Related codes

References: gsi.ext.gm.com, http:.