P066A
Cylinder 1 Glow Plug Control Circuit Low
Glow plugs (diesel)
Generic OBD-II
High
What it means
P066A means the engine computer has detected low voltage or a short to ground in the control circuit for the cylinder 1 glow plug on a diesel engine. Glow plugs preheat the combustion chamber so the engine can light off when cold, so this fault mainly shows up as hard cold starting, rough running until the engine warms, and white exhaust smoke.
High
Can I keep driving? Usually driveable in mild weather, but expect hard cold starts and rough cold-running until the glow plug circuit is repaired.
Common symptoms
What you'll notice
- Hard starting, especially in cold weather
- Rough running or a stumble right after a cold start
- White or blue exhaust smoke on a cold start
- Check-engine light (and often the wait-to-start/glow-plug light) on
Common causes
Most likely first
- Failed cylinder 1 glow plug (shorted or open element)Likely
- Short to ground or damaged wiring in the cylinder 1 glow plug control circuitCommon
- Corroded or loose connector at the glow plug or glow-plug control moduleCommon
- Faulty glow plug control moduleLess often
- Faulty ECM (least likely)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and confirm the fault is on cylinder 1 specifically.
- Measure the cylinder 1 glow plug resistance and replace it if it is open or out of spec.
- Inspect the glow plug wiring and connector for shorts to ground, chafing, and corrosion.
- Check the glow-plug control module outputs and its connections.
- Clear the code and retest with a cold start; if it returns, evaluate the control module and ECM.
FAQ
Quick answers
Is it safe to drive with P066A?+
In mild weather usually yes, but cold starts will be hard and the engine may run rough until warm. Fix it before winter to avoid a no-start.
Can I just clear P066A?+
You can, but it will return until the failed glow plug or its circuit is repaired — clearing does not fix the underlying fault.
Related codes
Often seen with P066A
Read & clear P066A yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX