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DTC codes / P06xx · Computer and output circuits / P066D

P066DCylinder 2 Glow Plug Control Circuit High

P066D means the engine control module (ECM) has detected a high-voltage condition on the glow plug control circuit for cylinder 2. The ECM expects the circuit to pull current and drop voltage when the glow plug is commanded on, but instead it sees voltage staying at or near battery voltage, indicating an open circuit, a failed glow plug, or a wiring fault.

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

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp, often with no other symptoms
  • Hard cold starting or extended cranking before the engine fires
  • Rough idle and white smoke from the exhaust immediately after a cold start
  • Reduced fuel economy during warm-up and increased exhaust emissions
  • In some vehicles, the engine may enter a limited-power mode or disable remote start

Common causes, most likely first

  1. Failed glow plug on cylinder 2 (open internal element)
  2. Open or damaged wiring between the glow plug control module and the cylinder 2 glow plug
  3. Corroded, loose, or disconnected electrical connector at the cylinder 2 glow plug
  4. Faulty glow plug control module that is not supplying current to the cylinder 2 circuit
  5. Blown fuse or open relay in the glow plug power supply circuit
  6. High-resistance connection at the glow plug harness or ground point

How to diagnose and fix it

  1. Inspect the cylinder 2 glow plug connector and harness for corrosion, looseness, or physical damage
  2. Measure the resistance of the cylinder 2 glow plug with a multimeter and compare it to the manufacturer's specification
  3. Check for battery voltage at the glow plug connector when the key is first turned on (glow plug commanded on)
  4. Test the glow plug control module output and the continuity of the wiring from the module to the glow plug
  5. Check the glow plug fuse and relay, and verify power and ground at the control module
  6. Replace the faulty glow plug or repair the open wiring as needed, then clear the code and verify the repair

What a high glow plug code really means

  • A high-voltage code on a glow plug circuit almost always indicates an open circuit, not a shorted glow plug, so replacing the glow plug without checking the wiring can be a wasted repair.
  • The glow plug control module often sets this code when the glow plug itself is fine but the harness connector has backed out or corroded, especially on vehicles with age-related terminal tension loss.
  • Some vehicles only run the glow plug self-test during a cold start, so the code may not reset immediately after repair until the engine is started from cold.
  • A weak battery or poor ground can cause the control module to misreport a high-voltage condition on one glow plug circuit, so always verify battery and ground integrity first.
  • This code does not always mean the glow plug is dead; a faulty control module driver for that cylinder can also set it, and swapping the glow plug to another cylinder is a quick diagnostic test.

Can I keep driving with P066D?

You can usually drive the vehicle, but cold starting will be harder and the engine may run rough or smoke until it warms up. Avoid long periods of idling from cold and have the fault repaired soon to prevent excessive wear and emissions problems.

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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