Short to voltage in the cylinder 1 glow plug control wiring; Damaged, corroded or loose connector at the glow plug or control module; Faulty cylinder 1 glow plug (internal fault affecting circuit feedback); Faulty glow plug control module or driver stage; Low battery voltage or poor power and earth supply to the glow plug system; Fault in the engine control unit; Outdated control unit software where the manufacturer has a known fault.

P066B

Cylinder 1 Glow Plug Control Circuit High.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
6B: FaultThe specific fault in that area

What it means

P066B is the generic code for a high signal in the cylinder 1 glow plug control circuit. The engine control unit (or glow plug control module) monitors the voltage or feedback on the circuit that drives the cylinder 1 glow plug. It saw a voltage higher than expected, or a fault status reported by the driver, so it set the code. Exact monitoring and wiring layout depend on the make, model and engine.

Symptoms

Glow plug or engine warning light on the dashboard
Hard starting or extended cranking on a cold engine
White smoke and rough running after a cold start
Misfire or uneven idle until the engine warms up
Increased emissions or poor cold-start performance
No noticeable symptoms when the engine is warm

Possible causes

Short to voltage in the cylinder 1 glow plug control wiringlikely
Damaged, corroded or loose connector at the glow plug or control modulelikely
Faulty cylinder 1 glow plug (internal fault affecting circuit feedback)possible
Faulty glow plug control module or driver stagepossible
Low battery voltage or poor power and earth supply to the glow plug systempossible
Fault in the engine control unitrare
Outdated control unit software where the manufacturer has a known faultrare

How to diagnose

Read all stored codes and freeze frame data, noting engine temperature and battery voltage when the fault set, and check for other glow plug codes
Check battery condition and charging voltage, as low or unstable voltage can upset glow plug diagnostics
Inspect the cylinder 1 glow plug wiring, connector and the control module connector for chafing, heat damage, corrosion, water and pushed-out pins
Check the wiring diagram for your model and engine, then test the control circuit for a short to voltage with the connector disconnected
Check the power and earth supplies to the glow plug control module, including fuses and relays
Measure the cylinder 1 glow plug resistance and compare it with the specification for the engine
Use a scan tool to activate the glow plugs and watch the commanded and actual status for cylinder 1
Check for applicable software updates or technical service bulletins for the vehicle

How to fix

Repair or replace damaged wiring that is shorted to voltage, and secure it away from heat and rubbing
Clean or replace corroded or damaged connectors and terminals
Replace the cylinder 1 glow plug if it is out of specification, following the correct torque procedure to avoid breaking it
Replace the glow plug control module if the driver stage is confirmed faulty
Restore correct battery, power and earth supply, and replace any faulty fuse or relay once the cause of failure is found
Update the control unit software only where the manufacturer lists it as a fix for this fault

Can I keep driving?

Low severity

The car can usually be driven, but cold starts may be hard and emissions may be higher. Book it in soon, particularly before cold weather, and avoid repeated long cranking.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.