P066A: cylinder 1 glow circuit low and the inrush check

P066A checks cylinder 1's glow-plug output for a short-to-ground condition in the documented system. The hardware-interface diagnosis evaluates electrical integrity during glow operation.

Low impedance and excessive inrush are distinct evidence

The table characterizes ground-short detection at 0.11 ohm and says the condition should not be detected above the 0.19-ohm minimum-load boundary. These are detection specifications, not a complete acceptable-resistance range for a replacement glow plug.

The source also allows an excessive inrush-current profile to set the ground-short condition. That inrush assessment occurs at key-on once per driving cycle. A later steady-state observation may therefore miss the event that supported the code.

Confirm an eligible assessment

The ordinary monitoring conditions include engine running outside cranking, battery voltage in range, no glow-system supply fault and duty cycle above 2.50% but below 97.50%. The filter is ten failures among twenty samples on a 100 ms task, with Type B reporting and glow-lamp indication.

For diagnosis, preserve whether the finding arose from inrush behavior or the operating circuit check, and identify cylinder 1 through the correct vehicle information. Verification must exercise the relevant valid event. The summary does not authorize external battery-power testing, identify a defective heating element independently of its circuit or prescribe glow-plug removal.

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 1232.