What it means
The control line for the glow plug on cylinder 2 is shorted to ground or the plug is internally shorted.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine controller switches and monitors the glow plug circuit for cylinder 2 to heat that cylinder for cold starting. P066C sets when cylinder 2's control circuit signal is below its expected electrical range, often because the circuit is shorted to ground or the plug is internally shorted.
Common causes
Most likely first.
- Control line shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Glow plug warning light on (often orange coil symbol)
- Hard cold start or no start in cold weather
- White smoke from exhaust during warm-up
How to diagnose it
- Record freeze-frame data and check whether the fault is current or intermittent, noting engine temperature and starting conditions.
- With the engine cool, inspect the cylinder 2 glow plug connector and harness for chafing, damaged insulation, moisture, or loose or corroded terminals.
- Compare the cylinder 2 glow plug and its wiring with the other cylinders for visible damage or signs of overheating.
- Test the cylinder 2 glow plug and its control wire for a short to ground using the vehicle's wiring diagram and manufacturer's specification.
- If the plug and wiring pass, test the glow plug control module output and its power and ground connections.
Common fixes
- Repair chafed cylinder 2 wiring that is shorting to ground.
- Clean or repair the cylinder 2 glow plug connector and terminals.
- Replace the cylinder 2 glow plug if it is internally shorted.
- Repair the glow plug control module or its power and ground connections if its output is faulty.
Can I keep driving?
In mild weather, the engine may still start and run, but a failed cylinder 2 glow plug can cause hard cold starts and white smoke during warm-up. In cold conditions, avoid relying on the vehicle until the fault is repaired.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.