P066A

Cylinder 1 Glow Plug Control Circuit Low

Q: 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.

Q: 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.

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.

Real-world scan data

Seen in

113

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P066A yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX