P066B
Cylinder 1 Glow Plug Control Circuit High
Glow plug / diesel preheat
Generic OBD-II
High
What it means
P066B is a diesel code meaning the engine computer saw an abnormally high voltage in the control circuit for cylinder 1's glow plug — usually an open plug, broken wiring, or a bad connection. Glow plugs preheat the cylinder for cold starts, so the main effect is hard cold starting and rough, smoky running until the engine warms.
High
Can I keep driving? Usually drivable — expect hard cold starts and rough, smoky running until the engine warms; a single glow plug rarely prevents starting outright.
Common symptoms
What you'll notice
- Hard starting when the engine is cold
- Rough running or misfire until the engine warms up
- White or grey smoke from the exhaust on a cold start
- Check-engine or glow-plug warning light on
Common causes
Most likely first
- Failed cylinder 1 glow plug (open or high resistance)Likely
- Open or shorted wiring in the glow plug control circuitCommon
- Corroded or loose connector at the glow plug or moduleCommon
- Faulty glow plug control moduleLess often
How to diagnose & fix
Work through it in order
- Scan and confirm the code, then note whether other cylinder glow-plug codes are present.
- Inspect the wiring and connector from the control module to the cylinder 1 glow plug for damage or corrosion.
- Measure the glow plug's resistance and compare to the others; replace it if open or out of spec.
- Test the control module output and its power/ground feeds if the plug and wiring are good. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P066B?+
Generally yes, but cold starts will be hard and the engine may run rough and smoky when cold. Fix it before winter to avoid harder starting.
Can I just clear P066B?+
It will come back as soon as the module tests the glow plug circuit again. The failed plug or wiring has to be repaired first.
Related codes
Often seen with P066B
Read & clear P066B 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