Faulty glow plug on cylinder 12 (open or shorted element); Damaged or corroded electrical connector at the cylinder 12 glow plug; Chafed or broken wire in the glow plug harness near the cylinder 12 location; Failed glow plug control module or relay affecting that circuit; High-resistance connection at the glow plug module or fuse box; Engine control module software or internal driver fault (less common)

DTC codes / P06xx · Computer and output circuits / P06C4

P06C4Cylinder 12 Glow Plug Circuit Range/Performance

Q: What does code P06C4 mean?

Cylinder 12 Glow Plug Circuit Range/Performance. P06C4 indicates that the engine control module has detected an out-of-range or performance fault in the glow plug circuit for cylinder 12, typically on a diesel engine. The computer monitors the circuit's electrical behavior and flags this code when the voltage or current does not match the expected profile during glow plug activation or self-test.

P06C4 indicates that the engine control module has detected an out-of-range or performance fault in the glow plug circuit for cylinder 12, typically on a diesel engine. The computer monitors the circuit's electrical behavior and flags this code when the voltage or current does not match the expected profile during glow plug activation or self-test.

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
Hard cold starting or extended cranking before the engine fires
Rough idle and white smoke immediately after a cold start
Reduced fuel economy during warm-up cycles
Often no symptoms besides the warning light, especially in mild weather

Common causes, most likely first

Faulty glow plug on cylinder 12 (open or shorted element)
Damaged or corroded electrical connector at the cylinder 12 glow plug
Chafed or broken wire in the glow plug harness near the cylinder 12 location
Failed glow plug control module or relay affecting that circuit
High-resistance connection at the glow plug module or fuse box
Engine control module software or internal driver fault (less common)

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm the fault is current and specific to cylinder 12
Inspect the cylinder 12 glow plug connector and harness for corrosion, looseness, or physical damage
Measure glow plug resistance with a multimeter and compare to the manufacturer's specification
Check for voltage and ground at the glow plug connector during the glow plug activation period
Test the glow plug control module output and related fuses if the circuit checks out
Replace the faulty glow plug or repair the harness as needed, then clear codes and verify the repair
Perform a cold-start test and re-scan to ensure the code does not return

What a glow plug code often hides

A glow plug can pass a simple resistance check when cold yet still fail to heat properly under load, so a performance code does not always mean the plug is completely open.
Corroded connectors and chafed harness sections near the cylinder head are frequent culprits that mimic a failed glow plug and are often overlooked during diagnosis.
The glow plug control module may set a cylinder-specific code even when the fault lies in a shared power or ground circuit, so always verify the entire circuit path.
On some engines, a single failed glow plug can trigger multiple glow plug codes, so do not assume every stored code points to a separate failed plug.
After replacing a glow plug, the engine control module may need a relearn or adaptation reset before the code clears permanently.

Can I keep driving with P06C4?

The vehicle is generally safe to drive, but cold starting may be difficult and prolonged cranking can stress the starter and battery. Avoid heavy load or high-speed driving until the fault is repaired, and address the issue before cold weather arrives.

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.