Failed glow plug on cylinder 4, typically an open heating element or a shorted internal coil; Damaged or corroded electrical connector at the cylinder 4 glow plug; Chafed, broken, or high-resistance wiring in the glow plug harness; Faulty glow plug control module or relay affecting individual cylinder output; Poor engine ground or corroded ground strap increasing circuit resistance; Moisture intrusion into the glow plug connector or control module housing; Aftermarket or incorrect-specification glow plug installed during a previous repair

DTC codes / P06xx · Computer and output circuits / P06C8

P06C8Cylinder 4 Glow Plug Incorrect

Q: What does code P06C8 mean?

Cylinder 4 Glow Plug Incorrect. P06C8 means the engine control module has detected an incorrect electrical condition on the glow plug circuit for cylinder 4, typically an open circuit, short to ground, or resistance outside the expected range. The module compares the glow plug's current draw or voltage feedback against a stored profile and flags the cylinder when the signal does not match.

P06C8 means the engine control module has detected an incorrect electrical condition on the glow plug circuit for cylinder 4, typically an open circuit, short to ground, or resistance outside the expected range. The module compares the glow plug's current draw or voltage feedback against a stored profile and flags the cylinder when the signal does not match.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illuminated check engine light, often with no other symptoms besides the warning light
Hard cold starting or extended cranking before the engine fires
Rough idle and white smoke from the exhaust immediately after a cold start
Misfire-like shudder or hesitation while the engine is warming up
A stored glow plug-related code alongside P06C8 in the module memory

Common causes, most likely first

Failed glow plug on cylinder 4, typically an open heating element or a shorted internal coil
Damaged or corroded electrical connector at the cylinder 4 glow plug
Chafed, broken, or high-resistance wiring in the glow plug harness
Faulty glow plug control module or relay affecting individual cylinder output
Poor engine ground or corroded ground strap increasing circuit resistance
Moisture intrusion into the glow plug connector or control module housing
Aftermarket or incorrect-specification glow plug installed during a previous repair

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm P06C8 is active and not a history code
Inspect the cylinder 4 glow plug connector and harness for corrosion, chafing, and loose pins
Measure glow plug resistance with a digital multimeter and compare it to the manufacturer's specification
Check the glow plug control circuit for voltage drop and continuity between the module and the plug
Test the glow plug control module output and verify the ground circuit is clean and tight
Replace the faulty glow plug or repair the damaged harness section, then clear codes and re-test from a cold start
Verify the repair with a cold-start glow plug activation test and confirm no codes return after several drive cycles

Why cylinder 4 glow plug codes mislead

A glow plug can pass a simple resistance check at room temperature yet still fail to draw correct current when the module commands it, so a load test or current clamp reading is more reliable than ohms alone.
The glow plug control module often stores individual cylinder codes when its own internal driver fails, so replacing the plug without testing module output can leave the code present.
Corrosion inside the connector boot is a frequent cause that looks like a failed plug on a meter but is resolved by cleaning and re-pinning the connector.
P06C8 can be set by a brief intermittent open circuit during a cold start, so a plug that tests good when warm may still be the culprit.
On engines with a shared glow plug feed, a single bad plug can drag down circuit voltage enough to set codes for neighbouring cylinders, so always check all glow plugs before condemning one.

Can I keep driving with P06C8?

The vehicle remains safe to drive because the glow plug only assists cold starting, but avoid long cranking periods and expect harder starts and more exhaust smoke in cold weather until the fault is repaired.

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.