Modern diesel glow systems switch each glow plug through its own power semiconductor (in a glow plug module or the engine module), so current and faults can be measured per plug; a faulty channel is switched off. CARB requires glow plugs to be checked for correct current and voltage drop and, since model year 2010, a single plug outside specification to be detected and named. P066C means the circuit to the cylinder 2 glow plug is shorted to ground or draws overcurrent. GM's engine-module drivers flag a circuit impedance to ground below 0.11 Ω (10 of 20 samples at 100 ms) with the check engine and glow plug lamps after two trips; Ford's 2023 Power Stroke checks during heating for more than 22 A for 0.3 s or 20–22 A for 2 s. The channel is switched off, so plug 2 no longer heats – harder cold starts and white smoke follow. A plug shorted internally is the usual cause; a wrong plug type is another.
Low severity. Can I keep driving?You can keep driving, although cold starts will be harder. Avoid cranking the starter repeatedly on cold days.
Cylinder 2 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine and glow plug lamp
- Harder cold start, especially in frost
- Rough running and white smoke right after a cold start
- Possibly blown glow system fuse
Most likely causes
- Glow plug cylinder 2 shorted internally (element against body)
- Plug wire insulation damaged and shorted to ground (block, bracket)
- Wrong plug type (resistance or voltage rating)
- Driver in the glow or engine module faulty, or software (GM 3.0 L 2020)
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P066C means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 6C: Specific fault in that system
Frequently asked questions
Can I keep driving with code P066C?
You can keep driving, although cold starts will be harder. Avoid cranking the starter repeatedly on cold days.
What causes code P066C?
The most common causes are: glow plug cylinder 2 shorted internally (element against body); plug wire insulation damaged and shorted to ground (block, bracket); wrong plug type (resistance or voltage rating).
Related codes
- P066DCylinder 2 Glow Plug Control Circuit High
- P0672Cylinder 2 Glow Plug Circuit/Open
- P06BACylinder 2 Glow Plug Circuit Range/Performance
- P06C6Cylinder 2 Glow Plug Incorrect
- P0670Glow Plug Control Module 1 Control Circuit/Open
- P064CGlow Plug Control Module 1
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 25OBDG04A
- gsi.ext.gm.com: 16OBDG04 Engine Diagnostics
- gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.