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
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