P06BC: Cylinder 4 Glow Plug Circuit Range/Performance
What to do
Ask for a voltage drop test on the glow plug supply rather than just a continuity check. They are not the same — a circuit can show perfect continuity and still lose too much voltage once the plugs actually draw current, and this is the last plug on the supply run, so it feels that loss most.
Ask for all four plugs to be done if any are coming out. Same labour, same risk of a plug snapping in the head, and they are the same age.
Expect hard starting and white smoke on cold mornings until it is repaired.
Mention whether the plugs have been replaced before, and whether they were original parts.
What this code means
Cylinder 4's glow plug circuit is electrically valid but not behaving correctly.
On a four-cylinder diesel this is the last plug in the row, and that position carries a specific and frequently missed cause.
Glow plugs are fed by a supply strap or harness that runs the length of the head, branching to each plug in turn. Every connection along that run is in series with the last plug on it — so cylinder 4 accumulates the voltage drop of the whole chain. A little corrosion at each of three earlier terminals is invisible individually and adds up here.
Which is why a voltage drop test along the supply, taken while the plugs are actually drawing current, is the measurement that matters. A continuity check confirms a path exists and says nothing about how much voltage that path loses under a heavy glow plug load.
This is the same failure as a corroded battery terminal that cranks fine until a cold morning. The circuit works until it is asked to carry real current — which, for a glow plug, is exactly when it matters.
Where the supply is sound, the plug itself is the honest answer. It is at the end of the row, it has heat-cycled as many times as the others, and it is due.
Do the set if any come out. The labour and the snapping risk are the same for one as for four.
Likely causes
Start here
- High resistance in sensor wiring · moderate
- Failed glow plug · moderate
- Corroded or loose connector · easy
Less common
- Broken glow plug supply wire · moderate
- Incorrect glow plugs fitted · moderate
Symptoms
Components involved
Information, not instruction
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