P066C: Cylinder 2 Glow Plug Control Circuit Low
What to do
Ask for all the glow plugs to be tested, not just the one this code names. They age together and access is most of the work — replacing them as a set avoids repeating a job that carries a real risk of a plug snapping off in the cylinder head.
Discuss that risk before work starts. A seized glow plug that breaks turns a routine job into a much larger one, so a garage proposing penetrating oil and heat cycles first is being careful.
Expect hard starting and white smoke on cold mornings until it is fixed.
Ask for the glow plug control module to be checked as well, since a shorted plug can damage its driver.
What this code means
The control circuit for cylinder 2's glow plug is sitting lower than commanded.
The mechanism is the family's — a short to ground, from a failed plug element, a chafed supply strap, or a driver already damaged.
What is worth adding here is the numbering, because this family does not run consecutively and that catches people out. The per-cylinder control circuit codes step in twos — P066A, P066C, P066E for cylinders 1 to 3 — and then the fourth cylinder appears at P067A rather than continuing the pattern. The intervening numbers belong to the high-side versions of the same circuits.
So never infer a cylinder's code number from its neighbour in this block. Check the number against the cylinder rather than counting from one you already know — it is a documented way to end up diagnosing the wrong cylinder entirely.
The practical advice for the repair is unchanged and worth repeating: glow plugs seize into the cylinder head and can snap off on removal. They also age as a set, so where access is most of the labour, replacing all of them in one visit is usually the better decision.
Measure the plug before the module is replaced. A plug that has gone short circuit can destroy its driver, and a new module fitted without finding it will fail the same way.
Likely causes
Start here
- Failed glow plug · moderate
- Wiring shorted to ground · moderate
- Corroded or loose connector · easy
Less common
- Broken glow plug supply wire · moderate
- Failed glow plug control module · moderate
Symptoms
Components involved
Information, not instruction
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