P066A: Cylinder 1 Glow Plug Control Circuit Low
What to do
Expect hard starting in cold weather, white smoke and rough running for the first minute or two, since this cylinder is not being pre-heated.
Understand that glow plug removal carries a genuine risk. These plugs corrode into the cylinder head and can snap off when removed — if that happens, the job becomes much larger and can involve removing the head. A garage that wants to soak the plugs and warm the engine first is protecting you, not padding the bill. It is worth discussing this before work starts.
Ask for all the glow plugs to be tested while the work is being done. They age together, access is most of the labour, and doing them as a set avoids repeating a risky job.
Ask for the glow plug control module to be checked too, since a shorted plug can damage it.
What this code means
The control circuit for cylinder 1's glow plug is sitting lower than commanded.
A glow plug control circuit reading low usually means a short to ground, and glow plugs have a specific way of getting there. The element sits inside the combustion chamber, and as it ages the sheath can crack or the element can fail short, dragging its driver down.
This matters more than an ordinary short because of what it can destroy. These circuits carry substantial current — a cold glow plug draws heavily for the first seconds. A shorted plug can take out its driver inside the glow plug control module, so the plug should be measured before any module is replaced.
The commonest causes in order:
- A glow plug that has failed short circuit, which is the plug rather than the wiring.
- The supply strap or wire shorting to the head or block. These run across a hot, tight space between plugs, and their insulation hardens with heat until it fails against earth.
- A driver stage already damaged inside the control module, which is often the consequence of the first cause.
Removing a seized glow plug is the genuine risk in this repair, and it is worth knowing before agreeing to it. They corrode into the head and snap off, at which point the job becomes an extraction and can mean removing the cylinder head. A garage that proposes penetrating oil and several heat cycles first is being careful, not slow.
Likely causes
Start here
- Failed glow plug · moderate
- Wiring shorted to ground · moderate
- Corroded or loose connector · easy
Less common
- Failed glow plug control module · moderate
- Broken glow plug supply wire · moderate
Symptoms
Components involved
Information, not instruction
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