What Does P066F Mean?
The PCM has detected a high voltage condition in the glow plug control circuit for cylinder 3 on a diesel engine. This indicates an open circuit in the cylinder 3 glow plug circuit, meaning no current is flowing to heat the glow plug. Without glow plug heating, cylinder 3 will not combust fuel efficiently during cold starts. The driver will experience harder starting in cold weather and white exhaust smoke from unburned diesel fuel during warm-up.
Common Causes
35%
Burned-out cylinder 3 glow plug (open heating element)
30%
Open circuit in cylinder 3 glow plug wiring or bus bar
20%
Loose or corroded terminal at cylinder 3 glow plug
10%
Glow plug control module cylinder 3 output failure
5%
Seized and broken glow plug requiring extraction
Diagnostic Steps
1
Disconnect the cylinder 3 glow plug and measure resistance — an infinite (OL) reading confirms a burned-out element. Spec is typically 0.5-2.0 ohms.
2
If the plug tests good, verify supply voltage at the bus bar or connector during the glow cycle. Use a test light or voltmeter — no voltage means the fault is upstream.
3
Inspect the bus bar segment feeding cylinder 3. These segmented bus bars can crack from vibration and thermal cycling. Look for hairline fractures or heavy oxidation on contact surfaces.
4
Check continuity from the glow plug control module output for cylinder 3 through to the glow plug connector. Repair any open circuit found in the harness.
5
If removing the glow plug, apply penetrating oil 24 hours in advance and warm the engine first. Cylinder 3 glow plugs on inline engines are often the most accessible, but still use a torque-limited extraction tool.
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 3 on a diesel engine. This indicates an open circuit in the cylinder 3 glow plug circuit, meaning no current is flowing to heat the glow plug. Without glow plug heating, cylinder 3 will not combust fuel effic...
The most common cause of P066F (Cylinder 3 Glow Plug Control Circuit High) is: Burned-out cylinder 3 glow plug (open heating element)
Typical repair costs for P066F range from $80 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Cold starting will be affected, particularly in sub-freezing temperatures. Once the engine is warm, operation is normal. Multiple failed glow plugs will compound the problem significantly. Repair before the onset of cold weather to prevent no-start conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P066F 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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