P039CCylinder 1 Pressure Variation Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- Some diesels have a combustion pressure sensor in the glow plug; the engine module uses it to control injection for each cylinder. This code concerns cylinder 1. GM's European 2.0 CDTI diesel (Saab 9-5, 2010) uses P039C as 'pressure variation too low', with three sub-checks, and sets it when the signal is outside its range for more than 0.2 s with the engine running; its fault table counts it among the signal performance faults together with the erratic code. What exactly is measured, Saab does not explain; presumably it is the swing of the pressure signal within an engine cycle – too small a swing means the signal barely responds, for example because the glow rod is stuck by carbon and no longer transmits the pressure, or because pressure build-up is missing. Usually there are no noticeable symptoms.
- What you may notice
- Check engine light on
- Usually no noticeable symptoms
- Possibly rough running or combustion noise because control is off
- Likely causes
- LikelyPressure-sensing glow plug faulty or coked (glow rod stuck)
- PossibleContact resistance in connector or ground
- RareActual compression loss or misfire in the cylinder
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €100 – €450
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.