What Does P066D Mean?
The PCM has detected a high voltage condition in the glow plug control circuit for cylinder 2 on a diesel engine. This indicates an open circuit condition preventing current from reaching the cylinder 2 glow plug. The glow plug is not heating, so cylinder 2 will not fire efficiently during cold starts until the engine reaches operating temperature. The driver will notice harder cold starts and white smoke from unburned fuel in that cylinder.
Common Causes
35%
Burned-out cylinder 2 glow plug (open element)
30%
Open circuit in cylinder 2 glow plug wiring or bus bar
20%
Loose or corroded connection at glow plug terminal
10%
Glow plug control module cylinder 2 driver open
5%
Damaged glow plug that has seized and broken internally
Diagnostic Steps
1
Disconnect the cylinder 2 glow plug connector and measure resistance across the glow plug terminals. An infinite (OL) reading confirms a burned-out element.
2
If the glow plug tests good (0.5-2.0 ohms), check for supply voltage at the bus bar or connector during the glow cycle with key on. No voltage indicates a wiring or module issue.
3
Inspect the bus bar connection at cylinder 2 — check for a loose nut, broken bus bar segment, or heavy corrosion on the terminal. These connections are subject to extreme heat cycling.
4
Trace the wiring from the glow plug control module to the cylinder 2 connection, checking for continuity. Focus on areas where the harness passes near moving parts or high-heat zones.
5
Before removing a seized glow plug, apply penetrating oil and use a torque-limited glow plug socket. Broken glow plugs in the cylinder head require professional extraction.
Estimated Repair Cost
$80 - $500
Parts + labor, varies by vehicle and location
The PCM has detected a high voltage condition in the glow plug control circuit for cylinder 2 on a diesel engine. This indicates an open circuit condition preventing current from reaching the cylinder 2 glow plug. The glow plug is not heating, so cylinder 2 will not fire efficiently during cold star...
The most common cause of P066D (Cylinder 2 Glow Plug Control Circuit High) is: Burned-out cylinder 2 glow plug (open element)
Typical repair costs for P066D range from $80 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Primarily a cold-start concern. With one glow plug out, the engine may start harder in cold weather and produce white smoke during warm-up. Once at operating temperature, the engine runs normally. In extreme cold, multiple non-functioning glow plugs could prevent starting entirely.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P066D 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
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