Failed glow plug on cylinder 8 (open or shorted element); Damaged or corroded electrical connector at the cylinder 8 glow plug; Broken or chafed wiring in the glow plug circuit for cylinder 8; Faulty glow plug control module (GPCM) or relay affecting only cylinder 8; Poor ground connection for the glow plug circuit; ECM internal driver fault for the cylinder 8 glow plug circuit (rare)

DTC codes / P06xx · Computer and output circuits / P0678

P0678Cylinder 8 Glow Plug Circuit

Q: What does code P0678 mean?

Cylinder 8 Glow Plug Circuit. P0678 means the engine control module (ECM) has detected a fault in the electrical circuit for the glow plug on cylinder 8 of a diesel engine. The ECM monitors the glow plug circuit for expected current flow and resistance, and this code sets when the readings fall outside the normal range for that specific cylinder.

P0678 means the engine control module (ECM) has detected a fault in the electrical circuit for the glow plug on cylinder 8 of a diesel engine. The ECM monitors the glow plug circuit for expected current flow and resistance, and this code sets when the readings fall outside the normal range for that specific cylinder.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illuminated 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
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 8 may also be stored

Common causes, most likely first

Failed glow plug on cylinder 8 (open or shorted element)
Damaged or corroded electrical connector at the cylinder 8 glow plug
Broken or chafed wiring in the glow plug circuit for cylinder 8
Faulty glow plug control module (GPCM) or relay affecting only cylinder 8
Poor ground connection for the glow plug circuit
ECM internal driver fault for the cylinder 8 glow plug circuit (rare)

How to diagnose and fix it

Inspect the glow plug connector and wiring for cylinder 8 for corrosion, damage, or looseness
Measure the resistance of the cylinder 8 glow plug and compare it to the manufacturer's specification
Check for battery voltage at the glow plug connector during the glow plug pre-heat cycle
Test the glow plug control module or relay for proper operation on the cylinder 8 circuit
Perform a voltage drop test on the ground circuit for the glow plug
Replace the faulty glow plug if resistance is out of specification
Repair or replace damaged wiring or connectors, then clear the code and verify the repair

Don't assume the glow plug is bad

A corroded connector or chafed wire often mimics a failed glow plug, so always inspect the circuit before replacing parts.
The glow plug control module may fail on a single channel, causing a cylinder-specific code even when all glow plugs are good.
P0678 can set intermittently due to a poor ground connection that only acts up in cold or damp conditions.
On some vehicles, a faulty glow plug can cause a misfire code for the same cylinder, leading to misdiagnosis as a fuel or compression issue.
Always verify the glow plug part number and voltage rating match the vehicle's original specification, as incorrect plugs can cause repeat failures.

Can I keep driving with P0678?

You can generally continue driving with P0678, but cold starting may be difficult and the engine may run rough until warm. Avoid prolonged driving with a known misfire, and address the issue before winter to prevent starting 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.