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DTC codes / P06xx · Computer and output circuits / P066C

P066CCylinder 2 Glow Plug Control Circuit Low

P066C means the engine computer has detected an abnormally low voltage or current on the control circuit for cylinder 2's glow plug, which is used to heat the combustion chamber for cold starts in diesel engines. The computer expects a certain electrical load when it commands the glow plug on, and a low reading indicates an open circuit, short to ground, or failed glow plug.

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 during warm-up
  • In severe cases, a no-start condition in cold weather

Common causes, most likely first

  1. Failed glow plug on cylinder 2 (open internal element)
  2. Damaged or corroded electrical connector at the cylinder 2 glow plug
  3. Broken or chafed wire in the glow plug control circuit
  4. Faulty glow plug control module (GPCM) output for cylinder 2
  5. Poor ground connection for the glow plug circuit
  6. Corrosion or damage at the GPCM connector pins

How to diagnose and fix it

  1. Clear the code and verify it returns; note any other stored codes
  2. Visually inspect the cylinder 2 glow plug connector and wiring for damage or corrosion
  3. Measure the resistance of the cylinder 2 glow plug with a multimeter and compare to specifications
  4. Check for battery voltage and ground at the glow plug connector during the glow plug on-time
  5. Test the control circuit from the GPCM to the glow plug for continuity and shorts to ground
  6. If the circuit is intact, substitute a known-good glow plug or test the GPCM output with a load
  7. Replace the faulty glow plug or control module as needed, then clear codes and verify repair

Glow plug circuit checks that save time

  • A glow plug can test good with an ohmmeter when cold but fail to draw current when hot, so always verify current draw with a clamp meter if resistance looks normal.
  • The GPCM often stores this code when the glow plug is actually fine but the connector has high resistance from corrosion, so inspect the pins before condemning the plug.
  • P066C is sometimes set by a weak battery or charging system fault that causes low voltage during the glow plug self-test, so check battery health first.
  • On some engines, the glow plug control module is integrated into the engine control module (ECM), making diagnosis of the output circuit more involved and expensive.
  • Do not overlook a poor engine ground strap, which can cause low voltage on the glow plug circuit and set this code intermittently.

Can I keep driving with P066C?

You can usually drive the vehicle, but cold starting may be difficult and prolonged driving with a failed glow plug can lead to carbon buildup and further engine damage. Avoid heavy load or high-speed driving until the fault is repaired, and address it promptly if you live in a cold climate.

Tools that help

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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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