P066A code meaning
Cylinder 1 Glow Plug Control Circuit Low
Powertrain system Generic code P06xx codes Check engine light
What P066A 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. P066A means the circuit to the cylinder 1 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 1 no longer heats – harder cold starts and white smoke follow. A plug shorted internally is the usual cause; a wrong plug type is another.
P066A 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 P066A
- 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
Common causes of P066A
- Most likelyGlow plug cylinder 1 shorted internally (element against body)
- PossiblePlug wire insulation damaged and shorted to ground (block, bracket)
- Less likelyWrong plug type (resistance or voltage rating)
- Less likelyDriver in the glow or engine module faulty, or 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 P066A 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. |
| 6A | The exact fault in that area. |
What to do next
- Note P066A 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 P066A
What does P066A mean?
P066A means "Cylinder 1 Glow Plug Control Circuit Low". 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 P066A?
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 P066A fail an emissions test?
Yes, it can. P066A 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 P066A 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 P066A 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.