Failed cylinder 1 glow plug (open circuit or internal short); Damaged, chafed or corroded wiring between the control module and the cylinder 1 glow plug; Corroded, loose or contaminated connector or glow plug terminal at cylinder 1; Fault in the glow plug control module or its output driver for cylinder 1; Poor earth or low supply voltage to the glow plug system, such as a weak battery or bad power feed or fuse; Wiring short to earth caused by heat damage or previous engine work; Outdated or faulty control module software, where a known issue exists for that engine.

P066A

Cylinder 1 Glow Plug Control Circuit Low.

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
6A: FaultThe specific fault in that area

What it means

P066A is the generic code for a low signal in the cylinder 1 glow plug control circuit. The engine control unit or glow plug control module monitors the circuit that drives the cylinder 1 glow plug and saw a voltage or current lower than expected, which points to a short to earth, high resistance or an open circuit in that circuit. Exact monitoring methods and circuit layout vary by manufacturer and engine.

Symptoms

Hard or extended cranking on cold starts
White or grey smoke at start-up until the engine warms
Rough running or misfire on cold start that clears when warm
Glow plug or engine warning light on the dash
Slightly higher emissions or fuel use when cold
Fault stored with no obvious change in drivability when warm

Possible causes

Failed cylinder 1 glow plug (open circuit or internal short)likely
Damaged, chafed or corroded wiring between the control module and the cylinder 1 glow pluglikely
Corroded, loose or contaminated connector or glow plug terminal at cylinder 1likely
Fault in the glow plug control module or its output driver for cylinder 1possible
Poor earth or low supply voltage to the glow plug system, such as a weak battery or bad power feed or fusepossible
Wiring short to earth caused by heat damage or previous engine workpossible
Outdated or faulty control module software, where a known issue exists for that enginerare

How to diagnose

Read all stored codes and freeze frame data, noting coolant temperature, battery voltage and whether other glow plug codes are present
Check battery condition and charging voltage, and inspect related fuses and relays per the vehicle wiring diagram
Visually inspect the glow plug wiring, connector and terminal at cylinder 1 for chafing, heat damage, corrosion and looseness
Measure the cylinder 1 glow plug resistance or current draw and compare with the manufacturer's specification, as values vary by engine
Check the control circuit wiring for continuity, short to earth and short to supply between the control module and the plug
Check supply and earth at the glow plug control module while the system is commanded on
Compare with other cylinders' glow plug readings to see whether the fault is isolated to cylinder 1
Check for applicable manufacturer service bulletins or software updates before condemning the control module

How to fix

Replace the cylinder 1 glow plug if it is open circuit, shorted or out of specification, using care to avoid snapping a seized plug
Repair or replace damaged wiring, and clean or replace corroded connectors and terminals
Restore good supply and earth connections and correct any fuse or relay fault
Replace the glow plug control module only after wiring and plug faults are ruled out
Update the control module software if an applicable update exists for a known fault

Can I keep driving?

Low severity

The car can normally be driven, but cold starting may be poorer, especially in cold weather. Book it in soon, as a failed plug can be hard to remove if left and wiring faults may worsen.

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