P066E: Cylinder 3 Glow Plug Control Circuit Low
What to do
Ask how many cylinders have set glow plug faults. If several have, the problem is usually the shared supply or earth to the glow plug module rather than several failed plugs — which is a much smaller repair, and it is worth establishing before four plugs are ordered.
Ask for all the plugs to be tested if any are being replaced. They age together, and access is most of the labour.
Discuss the risk of a plug snapping off in the cylinder head before work begins. It is a genuine possibility on an older engine and turns a routine job into a large one.
Expect hard starting and white smoke on cold mornings until it is repaired.
What this code means
The control circuit for cylinder 3's glow plug is sitting lower than commanded.
The causes are the family's, and the useful thing to add at this point is how to read several of these codes together.
One cylinder low points at that plug or its own wire. All cylinders low at once almost never means every plug failed — it points at something shared: the supply feed to the control module, its earth, or the module's own power stage. Glow plugs draw heavy current through a common supply, so one bad connection starves all of them.
A car with four glow plug codes has one fault far more often than four, and starting with the module's supply and earth is much cheaper than starting with the plugs.
Where only this cylinder is affected, the plug is the likely answer — but measure rather than assume, because the wiring between the module and each plug runs across a hot, cramped space where insulation hardens and fails against the head.
Cylinder 3 sits in the middle of an inline engine, where it runs hottest, which is a real reason for its plug and its wiring to fail first rather than a coincidence.
The removal risk applies as it does across the family. Seized plugs snap; plugs age as a set; access is most of the labour.
Likely causes
Start here
- Failed glow plug · moderate
- Wiring shorted to ground · moderate
Less common
- Poor sensor ground connection · moderate
- Failed glow plug control module · moderate
- Broken glow plug supply wire · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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