P06BB: Cylinder 3 Glow Plug Circuit Range/Performance
What to do
Ask for all the glow plugs to be tested together. This cylinder is in the hottest part of the engine, so its plug tends to go first — but the others are usually not far behind, and access is most of the labour.
If the engine is rough only when cold and there are also injection timing faults stored, ask for a compression test on this cylinder. Low compression produces very similar cold symptoms to a weak glow plug, and it is worth ruling out before spending on parts.
Ask for the connector and supply strap to be inspected first, since corrosion mimics a failing plug exactly.
Expect hard starting and white smoke on cold mornings.
What this code means
Cylinder 3's glow plug circuit is electrically valid but not behaving correctly.
The module infers plug temperature from resistance, and this plug's resistance no longer fits.
Cylinder 3 sits in the hot middle of an inline block, and for glow plugs specifically that has a consequence worth knowing: the plug and its supply strap both live in the hottest, most cramped part of the engine. Insulation hardens there first, terminals oxidise there first, and the plug itself heat-cycles hardest.
So a middle-cylinder glow plug code on an inline engine is usually the beginning of a set rather than an isolated failure — which supports doing them together rather than one at a time.
There is a second reason to look at the whole picture on this cylinder. A weak glow plug and low compression produce very similar cold-running symptoms, and this block contains codes for one while the timing blocks contain codes for the other. A cylinder that is rough only when cold, with both a glow plug code and an injection timing code, is worth a compression test before parts are ordered — because low compression on that cylinder can produce both.
The glow plug is the cheaper thing to eliminate, so start there — but do not stop there if the fault persists.
Likely causes
Start here
- Failed glow plug · moderate
- High resistance in sensor wiring · moderate
- Corroded or loose connector · easy
Less common
- Low engine compression · specialist
- Incorrect glow plugs fitted · moderate
Symptoms
Components involved
Information, not instruction
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