P066E: cylinder 3 glow circuit low versus excessive initial current

P066E evaluates the electrical integrity of cylinder 3's glow-plug output for a ground-short condition. The source includes both an impedance-based description and an inrush-overcurrent route to the same fault category.

The timing of the failure is useful evidence

The inrush-profile check occurs at key-on once per driving cycle. That makes an initial event diagnostically different from a low-impedance condition observed during subsequent controlled glow operation. Capturing only steady behavior can omit the relevant initial event.

The listed impedance detection point is 0.11 ohm, while the table says a ground short should not be detected when circuit impedance is above 0.19 ohm. These limits do not define every acceptable replacement component or a safe live-circuit resistance test.

Verify with the enabling gates satisfied

Ordinary monitoring requires engine running without cranking, valid battery voltage, no glow-supply fault, enabled diagnostics and duty cycle above 2.50% and below 97.50%. The filter is ten failures among twenty samples on a 100 ms task. Type B reporting and glow-lamp indication are listed.

Verification should use the appropriate cylinder-3 service procedure and reproduce the relevant valid event. The summary does not identify the short's physical location or prove a failed plug independently of its wiring and driver. It provides no instruction to bypass glow control or connect the element directly to the battery.

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 1236.