P066F: cylinder 3 glow circuit high has two load-dependent cases

The P066F monitor identifies a short-to-high-voltage condition on cylinder 3's glow output. GM's table describes separate hardware detection cases for two load-resistance bands.

Keep the load and short path separate

With load resistance above 0.65 ohm, a power-source path below 0.5 ohm is the documented detection case. With load between 0.2 and 0.65 ohm, the path boundary is below 0.14 ohm. The power source may be within the normal or high operating-voltage range.

The load resistance and the unwanted path resistance are different quantities. Treating either short-path threshold as the plug's normal specification would lose that distinction and could lead to an unsupported replacement decision.

What constitutes a valid check

The source requires engine operation outside cranking, battery voltage in range, no glow-supply fault and enabled diagnostics. Ten failed samples among twenty at 100 ms intervals constitute the filter; Type B reporting and glow-lamp indication are listed.

Identify the correct cylinder-3 output, establish supply validity and apply the relevant service procedure to evaluate the circuit. A normal result while this monitor is inhibited is inconclusive. The table does not locate a fault within wiring, the load or the controller, nor does it authorize energized resistance testing, external battery application or bypassing driver protection.

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 1237.