What Does P066A Mean?
The PCM has detected a low voltage condition in the glow plug control circuit for cylinder 1 on a diesel engine. Glow plugs heat the combustion chamber to aid cold starting and reduce white smoke during warm-up. A low circuit condition typically indicates a short to ground in the control wire or a failed glow plug drawing excessive current. The driver may experience hard starting in cold weather and excessive white smoke from the affected cylinder.
Common Causes
35%
Short to ground in cylinder 1 glow plug control wiring
30%
Failed glow plug with internal short (low resistance)
20%
Corroded or damaged glow plug connector or bus bar
10%
Glow plug control module driver circuit fault
5%
Damaged wiring harness near exhaust manifold
Diagnostic Steps
1
Disconnect the glow plug connector or bus bar for cylinder 1 and measure glow plug resistance — spec is typically 0.5-2.0 ohms for most modern diesel glow plugs. A reading near 0 ohms indicates an internal short.
2
With the glow plug disconnected, check for continuity between the control wire and engine ground. Any continuity confirms a short to ground in the harness.
3
Inspect the glow plug connector, bus bar, and wiring for heat damage, corrosion, or melted insulation — these components are in a high-heat environment near the exhaust manifold.
4
Measure the supply voltage from the glow plug control module with the glow plug cycle active (key on, engine off) — should be battery voltage or a PWM-controlled voltage depending on system type.
5
If wiring is good, swap the cylinder 1 glow plug with an adjacent cylinder and clear codes to see if the fault follows the plug, confirming a bad glow plug.
Estimated Repair Cost
$80 - $400
Parts + labor, varies by vehicle and location
The PCM has detected a low voltage condition in the glow plug control circuit for cylinder 1 on a diesel engine. Glow plugs heat the combustion chamber to aid cold starting and reduce white smoke during warm-up. A low circuit condition typically indicates a short to ground in the control wire or a f...
The most common cause of P066A (Cylinder 1 Glow Plug Control Circuit Low) is: Short to ground in cylinder 1 glow plug control wiring
Typical repair costs for P066A range from $80 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle may be difficult to start in cold weather and may produce excessive white smoke. Once running, the engine should operate normally. Not an immediate safety risk, but cold-start issues could leave you stranded in winter conditions. Repair before cold weather.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P066A 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.
