What Does P066B Mean?
The PCM has detected a high voltage condition in the glow plug control circuit for cylinder 1 on a diesel engine. This indicates an open circuit, preventing current from flowing to the glow plug. The cylinder 1 glow plug is not heating, which will cause that cylinder to misfire during cold starts until the engine warms up. The driver will notice hard starting in cold conditions, rough running for the first few minutes, and white smoke from the exhaust.
Common Causes
30%
Open circuit in cylinder 1 glow plug wiring or connector
30%
Burned-out glow plug with open heating element
20%
Corroded or loose bus bar connection at the glow plug
15%
Glow plug control module not supplying power (open driver)
5%
Blown glow plug fuse or fusible link
Diagnostic Steps
1
Disconnect the cylinder 1 glow plug connector and measure glow plug resistance — an infinite (OL) reading confirms a burned-out element requiring replacement.
2
If the glow plug resistance is within spec (0.5-2.0 ohms), check for voltage at the glow plug connector during the glow cycle (key on, engine off). No voltage indicates an open in the supply circuit.
3
Inspect the bus bar connections at all glow plugs — these are subject to high heat and vibration. Look for loose nuts, broken bus bar links, and heavy corrosion. Torque to spec (typically 10-15 Nm).
4
Check the glow plug relay/fuse and the main glow plug power circuit for continuity from the control module to cylinder 1.
5
If all external circuits are intact, check glow plug control module output using bidirectional control on the scan tool while monitoring voltage at the module connector.
Estimated Repair Cost
$80 - $400
Parts + labor, varies by vehicle and location
The PCM has detected a high voltage condition in the glow plug control circuit for cylinder 1 on a diesel engine. This indicates an open circuit, preventing current from flowing to the glow plug. The cylinder 1 glow plug is not heating, which will cause that cylinder to misfire during cold starts un...
The most common cause of P066B (Cylinder 1 Glow Plug Control Circuit High) is: Open circuit in cylinder 1 glow plug wiring or connector
Typical repair costs for P066B range from $80 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Hard starting in cold weather is the primary concern — the engine may crank excessively or fail to start in sub-freezing temperatures. Once warm, the engine runs normally. White smoke during warm-up is expected with a non-functioning glow plug. Repair before winter weather to avoid being stranded.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P066B to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Glow Plug System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
