P2538: accessory-input diagnosis includes the network power-mode state

P2538 is an accessory-position circuit-high diagnosis, but this GM implementation is not described as a simple voltmeter limit. The controller evaluates its accessory-input wake-up status together with terminal-15 status and a network power-mode message.

Read the input in its operating context

The published fault combination includes active terminal-15 status, a nonzero accessory-pin wake-up state and a CAN state identified as a crank request. The comparison is listed for one second, once per driving cycle. A high accessory input during a different operating state is not enough to reproduce this particular rule.

The paired low-circuit code on the page uses a zero accessory wake-up state and a network run state. Those differences make the exact code and message state useful evidence; grouping both faults under “bad ignition switch” discards the monitor's logic.

Preserve the transition rather than only the final state

A workshop should record the relevant power-mode and wake-up data around the original event. A normal-looking steady state after the engine starts may not explain what happened during the transition. The table provides no pin voltage, connector position or direct mechanical-switch replacement instruction.

Use the actual vehicle's wiring and power-mode procedure to distinguish an input issue from inconsistent module or network state information. Verification should establish the correct relationship during an eligible cycle and address associated faults. Clearing P2538 does not demonstrate that the accessory and network states agree when the same transition next occurs.

Source and applicability

GM 2020 20OBDG07 accessory-input monitor. Only the cited monitor and calibration are described; per-code vehicle and engine applicability has not been established.

20OBDG07 Diagnostics, physical PDF page 393.