P066C: cylinder 2 glow control ground-short monitoring

The documented P066C strategy evaluates cylinder 2's glow output for a short to ground. Hardware feedback, rather than a direct heating-performance measurement, supplies the fault decision.

Preserve the key-on event

A ground-short result can also arise when the glow system's inrush-current profile exceeds its specified behavior. That check is performed at key-on only once per driving cycle. A circuit that appears normal later in the cycle may not reproduce an earlier excessive-inrush finding.

For the impedance-based description, the source gives 0.11 ohm as the ground-short detection point and says no such detection should occur above 0.19 ohm. The intervening region is not defined here as a workshop acceptance band.

Operating prerequisites limit a retest

The ordinary check needs battery voltage in range, engine running outside cranking, no glow-supply fault, enabled diagnostics and duty cycle between 2.50% and 97.50%. Ten failures among twenty 100 ms samples meet the listed filter, with Type B reporting and glow-lamp indication.

Identify the cylinder-2 channel and determine whether inrush or operating evidence is relevant before choosing the service check. Verification should address that same eligible event. The table does not prove the heating element is the only possible cause and gives no safe procedure for direct powering, resistance measurement on a live circuit or 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 1234.