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. P066D means the circuit to the cylinder 2 glow plug is shorted to battery voltage, so the driver cannot switch it off. GM's engine-module drivers detect an impedance to the supply below 0.5 Ω (0.14 Ω for low-resistance plugs) in 10 of 20 samples, with the lamps after two trips; Ford's 2023 Power Stroke sees 0 A through its measuring path while the plug is commanded on (about 1 s). A permanently powered low-voltage plug can overheat and burn through, after which an open-circuit code follows, and the battery may drain.
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
- Plug permanently powered, possible battery drain
- Plug burns through, then an open-circuit code
- Often no symptom when warm
Most likely causes
- Cylinder 2 plug wire touching battery positive (chafing in the harness)
- Moisture or corrosion in the module or plug connector (leakage between pins)
- Driver in the glow or engine module shorted (permanently on)
- Glow module 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 P066D means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 6D: Specific fault in that system
Frequently asked questions
Can I keep driving with code P066D?
You can keep driving, although cold starts will be harder. Avoid cranking the starter repeatedly on cold days.
What causes code P066D?
The most common causes are: cylinder 2 plug wire touching battery positive (chafing in the harness); moisture or corrosion in the module or plug connector (leakage between pins); driver in the glow or engine module shorted (permanently on).
Related codes
- P066CCylinder 2 Glow Plug Control Circuit Low
- 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.