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DTC codes / P06xx · Computer and output circuits / P0677

P0677Cylinder 7 Glow Plug Circuit

P0677 indicates that the engine control module has detected an electrical fault in the glow plug circuit for cylinder 7 of a diesel engine. The module monitors the circuit for expected current flow and voltage characteristics, and when the readings fall outside the programmed range, it flags the circuit as faulty and illuminates the glow plug warning light.

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

Symptoms you may notice

  • Illumination of the glow plug or check engine light, often with no other noticeable symptoms
  • Hard starting or extended cranking when the engine is cold
  • Rough idle or misfire immediately after a cold start that may smooth out as the engine warms
  • Excessive white smoke from the exhaust during cold running
  • Reduced fuel economy and increased emissions during warm-up
  • In severe cases, a misfire code for cylinder 7 may also be stored

Common causes, most likely first

  1. Failed glow plug on cylinder 7 (open or shorted heating element)
  2. Damaged or corroded electrical connector at the cylinder 7 glow plug
  3. Broken, chafed, or open wiring in the glow plug circuit for cylinder 7
  4. Faulty glow plug control module or relay affecting only that circuit
  5. Poor ground connection for the glow plug circuit
  6. Engine control module internal driver fault for that circuit (rare)

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related glow plug or misfire codes
  2. Inspect the cylinder 7 glow plug connector and wiring for corrosion, looseness, or damage
  3. Measure the resistance of the glow plug with a multimeter and compare it to the manufacturer's specification
  4. Check for battery voltage and ground at the glow plug connector during the glow plug preheat cycle
  5. Test the glow plug control module output for that cylinder if the circuit tests good
  6. Replace the faulty glow plug or repair the wiring as needed
  7. Clear the code and perform a cold-start drive cycle to confirm the repair

Glow plug circuit faults that fool technicians

  • A glow plug can fail intermittently when hot, so a resistance check on a warm engine may pass even though the plug is faulty when cold.
  • Corrosion inside the connector boot often mimics a failed glow plug, so always inspect and clean the terminals before condemning the plug.
  • The glow plug control module may set a circuit code for one cylinder when the actual problem is a shared power or ground feed affecting multiple cylinders.
  • Some vehicles use a glow plug module that communicates over a data bus, and a module fault can set P0677 without any wiring or plug issue.
  • Replacing a glow plug without checking the control module's output can lead to repeat failures if the module is overdriving the circuit.

Can I keep driving with P0677?

You can usually drive the vehicle, but cold starting will be harder and the engine may run rough until warm, so avoid heavy loads and long idling until the fault is repaired. Prolonged driving with a failed glow plug can damage the catalytic converter and increase emissions.

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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