What it means
The control line for the glow plug on cylinder 1 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 1 to warm the combustion chamber before and during a cold start. P066A sets when that cylinder's control circuit reads below its expected electrical range, often from a short to ground or a shorted glow plug.
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 1 glow plug connector and harness for chafing, damaged insulation, moisture, or loose or corroded terminals.
- Compare the cylinder 1 glow plug and its wiring with the other cylinders for visible damage or signs of overheating.
- Test the cylinder 1 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 1 wiring that is shorting to ground.
- Clean or repair the cylinder 1 glow plug connector and terminals.
- Replace the cylinder 1 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 vehicle may run with one inoperative glow plug, but expect harder cold starts and possible white smoke during warm-up. In cold conditions, avoid relying on it for essential trips because it may become difficult or impossible to start.
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.