P0381: wait-to-start lamp open-circuit monitoring

P0381 refers to the wait-to-start lamp circuit in the documented monitor. It checks the electrical integrity of the glow-plug/heater indicator output, rather than measuring whether a glow plug heats correctly.

The indicator is a separate diagnostic target

The hardware interface detects an open load through the impedance of the circuit attached to the controller output. The source also states that open-circuit detection is not required when the off-state diagnostic leakage-current source is disabled. This means the absence of an open-load report under that condition does not prove continuity.

The monitor requires a configured glow lamp, an enabled test, run/crank on, voltage above 11 V and an engine that is not cranking. These prerequisites explain why a vehicle without the relevant lamp configuration or an observation made during cranking is not a valid comparison.

What to check before drawing a repair conclusion

Establish whether the fault belongs to the indicator output and whether the lamp is configured as present. The generic “glow plug/heater” wording can otherwise divert attention toward the heating elements even though this row concerns the lamp circuit.

The table lists 10 failures among 15 samples and notes that open-load monitoring is implemented in hardware without additional debounce. It gives Type B, two-trip reporting. Verification should demonstrate correct lamp-circuit operation under the applicable test conditions; it does not follow that replacing glow plugs would resolve this code.

Source and applicability

2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately. These tables describe detection logic, not a complete repair procedure or a component replacement verdict. gm-2025-04b, PDF page 1180.