What it means
The control line for the glow plug on cylinder 3 is open or shorted to supply - the plug cannot be heated.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine controller commands the glow plug for cylinder 3 and checks the circuit's electrical response. P066F sets when the cylinder 3 control circuit reads above its expected range, which can indicate an open plug or wire or a control wire shorted to supply.
Common causes
Most likely first.
- Open in plug or wiring
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
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
- Save freeze-frame data and note whether the fault occurred during a cold start or glow plug operation.
- With the engine cool, inspect the cylinder 3 glow plug connector and wiring for a disconnected plug, broken conductor, corrosion, or insulation damage.
- Compare the cylinder 3 connection and harness routing with the other glow plug circuits to identify a local open or damaged wire.
- Using the vehicle wiring diagram, test the cylinder 3 plug and control wire for continuity and for a short to supply against the manufacturer's specification.
- If the plug and wiring pass, check the glow plug control module output and its power and ground connections.
Common fixes
- Reconnect or repair an open cylinder 3 glow plug wire.
- Repair corroded or damaged connector terminals.
- Replace the cylinder 3 glow plug if its element is open.
- Repair a wire shorted to supply or a faulty glow plug control module.
Can I keep driving?
The engine may run normally once started, but cylinder 3 may not heat properly for a cold start, causing hard starting or white smoke during warm-up. Avoid relying on the vehicle in cold weather until the fault is fixed.
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.