What it means
Same as P06F6 with the control line shorted to supply. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Glow plug control module 2 is commanded over a control circuit from the engine controller. P06F8 sets when that line is detected too high, commonly because it is shorted to a supply source or the circuit is otherwise being pulled high.
Common causes
Most likely first.
- Control line shorted to supply
- 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 scan for related glow-plug faults to establish whether the high control signal is persistent or intermittent.
- Inspect module 2's connector and harness for pinched insulation, exposed wire, corrosion, or contact with a power feed.
- Use the wiring diagram to test the control line for a short to supply, following the manufacturer's specified procedure.
- Check connector terminal fit and move the harness during testing to reveal an intermittent short or poor contact.
- If external wiring tests correctly, test module 2's control input and the engine-controller output using the manufacturer's diagnostic procedure.
Common fixes
- Repair the control wire where it is contacting a supply wire or source.
- Replace damaged connector terminals or wiring and secure the harness away from power feeds.
- Clean and reseal a connector affected by corrosion or moisture.
- Replace module 2 only if the high signal remains with the external circuit confirmed sound.
Can I keep driving?
The engine may still drive normally, but diesel preheating can be unreliable, making cold starts harder and increasing white smoke during warm-up. Avoid repeated prolonged cranking and arrange repair before cold weather.
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.