P066D
Cylinder 2 Glow Plug Control Circuit High
Glow plug / diesel preheat
Generic OBD-II
High
What it means
P066D is a diesel code: the ECM has detected a high-voltage (usually open-circuit) fault in the glow plug control circuit for cylinder 2. Glow plugs pre-heat the cylinder to help cold starting, so a fault here mainly shows up as hard cold starts and rough, smoky running until the engine warms up.
High
Can I keep driving? You can usually drive once the engine is warm, but expect hard cold starts and rough cold running until it's fixed.
Common symptoms
What you'll notice
- Hard or slow starting when the engine is cold
- Rough running and white smoke on a cold start until the engine warms up
- Glow plug (coil) warning light or check-engine light on
- Reduced power or hesitation in cold weather
Common causes
Most likely first
- Failed glow plug on cylinder 2Likely
- Open or shorted glow plug wiring/harness to cylinder 2Common
- Corroded or loose connector at the glow plugCommon
- Faulty glow plug control moduleLess often
- Faulty ECM/PCM driver (less common)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and confirm the code points to cylinder 2.
- Measure the cylinder 2 glow plug resistance and compare to the others — replace it if out of spec.
- Inspect the glow plug wiring and connector for opens, shorts, and corrosion.
- Check the glow plug control module's output and power/ground feeds.
- Repair or replace the faulty part, clear the code, and retest a cold start.
FAQ
Quick answers
Is it safe to drive with P066D?+
Usually yes once the engine is warm, but cold starts will be hard and it may run rough and smoke until warm. It won't damage the engine, but fix it before cold weather.
Can I just clear P066D?+
You can, but it returns once the ECM re-tests the glow plug circuit. The failed plug or wiring has to be repaired to clear it for good.
Related codes
Often seen with P066D
Read & clear P066D 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