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DTC codes / P06xx · Computer and output circuits / P066B

P066BCylinder 1 Glow Plug Control Circuit High

P066B indicates that the powertrain control module has detected an abnormally high voltage on the control circuit for cylinder 1's glow plug, which is used in diesel engines to warm the combustion chamber for reliable cold starts. The circuit is expected to operate within a specific voltage range, and a high reading typically points to an open circuit, a faulty glow plug, or a wiring issue that prevents proper current flow.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • Hard starting or rough idle when the engine is cold
  • Excessive white smoke from the exhaust after a cold start
  • Engine misfire or stumbling during the first few minutes of operation
  • Reduced fuel economy or increased emissions during warm-up
  • In rare cases, a no-start condition in very cold weather

Common causes, most likely first

  1. Failed glow plug with an open internal element
  2. Damaged or disconnected electrical connector at the glow plug
  3. Broken or corroded wire in the glow plug control circuit
  4. Faulty glow plug control module or relay
  5. Poor ground connection for the glow plug circuit
  6. Corrosion or water intrusion in the wiring harness
  7. PCM internal fault (uncommon)

How to diagnose and fix it

  1. Inspect the glow plug connector and wiring for visible damage, corrosion, or looseness
  2. Measure the resistance of the glow plug with a multimeter and compare to specification
  3. Check for voltage on the control circuit with the key on and engine cold
  4. Test the glow plug control module or relay for proper operation
  5. Repair or replace any damaged wiring or connectors
  6. Replace the faulty glow plug if resistance is out of range
  7. Clear the code and verify the repair with a scan tool

Glow plug circuit high voltage pitfalls

  • A high voltage code often results from an open circuit rather than a short, so a disconnected glow plug or broken wire is a common culprit.
  • The glow plug control module may set this code if it detects an open circuit during its self-test, even if the glow plug itself is good.
  • Corroded connectors can mimic a failed glow plug, so always inspect and clean terminals before replacing parts.
  • This code can be set by a faulty ground connection that affects the entire glow plug circuit, not just cylinder 1.
  • In some vehicles, the glow plug control module is integrated into the PCM, making diagnosis more complex and expensive.

Can I keep driving with P066B?

You can generally drive with this code, but cold starting may be difficult and prolonged driving with a faulty glow plug can lead to excessive smoke and carbon buildup. Avoid heavy loads or extended idling until the issue is repaired, and address it promptly to prevent further damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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