P039C Cylinder 1 Pressure Variation Low

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.

Likely causes

Symptoms

• Check engine light on
• Usually no noticeable symptoms
• Possibly rough running or combustion noise because control is off

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: saabwisonline.com, patents.google.com, law.cornell.edu.