P06B9: Cylinder 1 Glow Plug Circuit Range/Performance
What to do
If the glow plugs have been replaced recently, mention it and say whether they were original parts. Glow plugs are not interchangeable — they come in different resistances and voltages — and a plug of the wrong specification sets this fault while being brand new.
Ask for the connector and supply strap to be checked before the plug is condemned. Corrosion adds resistance that the module reads as a failing plug, and it is much cheaper to fix.
Expect hard starting and white smoke on cold mornings, and more so as the weather gets colder.
If plugs are being replaced, do the set and discuss the risk of one snapping off in the head first.
What this code means
Cylinder 1's glow plug circuit is electrically valid but not behaving correctly.
Range/performance is the more informative half of the glow plug family, and it works by measuring resistance. A glow plug's resistance rises predictably as it heats, so the control module infers the plug's temperature from the current it draws — that is how it controls each plug individually rather than simply switching them on.
This code says the resistance is wrong for the conditions. Not open, not shorted: the plug is not heating the way a healthy one would.
Which points at three things.
- A plug degrading with age. The element changes resistance as it wears, so the module can no longer predict its temperature. This is ordinary wear and the commonest cause.
- Resistance in the supply. A corroded terminal or a tired strap adds resistance in series with the plug, which the module cannot distinguish from the plug itself changing. The connector is worth eliminating first because it is cheap.
- The wrong plug fitted. Glow plugs are not interchangeable — different resistances and different voltages exist, and a plug of the wrong specification is out of range from the moment it is installed.
That last cause is worth asking about wherever plugs have been replaced recently, especially with non-original parts. It sets this code on brand new hardware.
Likely causes
Start here
- Failed glow plug · moderate
- High resistance in sensor wiring · moderate
- Corroded or loose connector · easy
Less common
- Incorrect glow plugs fitted · moderate
- Failed glow plug control module · moderate
Symptoms
Components involved
Information, not instruction
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