P066D code meaning
Cylinder 2 Glow Plug Control Circuit High
Powertrain system Generic code P06xx codes Check engine light
What P066D means
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.
P066D at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P066D
- 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
Common causes of P066D
- Most likelyCylinder 2 plug wire touching battery positive (chafing in the harness)
- PossibleMoisture or corrosion in the module or plug connector (leakage between pins)
- PossibleDriver in the glow or engine module shorted (permanently on)
- Less likelyGlow module software (GM 3.0 L 2020)
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Glow plugGlow plug wiringGlow plug control module
What each part of P066D means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 6D | The exact fault in that area. |
What to do next
- Note P066D and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P066D
What does P066D mean?
P066D means "Cylinder 2 Glow Plug Control Circuit High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P066D?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P066D fail an emissions test?
Yes, it can. P066D is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P066D to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P066D clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.