P037A: The glow indicator circuit is separate from the glow plugs themselves

P037A names the glow-plug or heater indicator control circuit low. In this GM implementation, the target is the wait-to-start lamp connection. A fault in this indicator circuit is not direct evidence that a particular engine glow plug has failed.

A lamp-circuit integrity check

The hardware interface evaluates a connection toward vehicle ground at the lamp-control pin. The row lists a 0.5 Ω reference for that electrical classification. It does not identify a connector terminal or authorize a resistance measurement with the controller powered.

The lamp must be configured as present, the test enabled, run/crank active and its supply above 11 V. Actual engine cranking is excluded. Those conditions explain why a reading obtained during cranking may not reproduce the event even though the supply circuit is named run/crank.

Pay attention to the diagnostic current source

The document says ground-short detection is not required when the off-state diagnostic leakage-current source is disabled. An absent failure indication in that state is therefore not proof that the lamp circuit has passed.

The table lists ten failures within fifteen samples and notes that ground-short monitoring is implemented in the hardware interface without further debounce. It does not give enough information to convert that into an invented elapsed-time rule. Retain the lamp configuration, command and monitoring state before attempting the applicable circuit-isolation procedure. The code alone does not justify replacing engine heating components.

Source and applicability

2025 GM 04A documented monitor only. Vehicle, engine and controller calibration applicability must be established separately; no model fitment is inferred from the file. GM PDF, physical page 942.