P039C code meaning
Cylinder 1 Pressure Variation Low
Powertrain system Generic code P03xx codes Check engine light
What P039C means
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.
P039C at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor1 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P039C
- Check engine light on
- Usually no noticeable symptoms
- Possibly rough running or combustion noise because control is off
Common causes of P039C
- Most likelyPressure-sensing glow plug faulty or coked (glow rod stuck)
- PossibleContact resistance in connector or ground
- Less likelyActual compression loss or misfire in the cylinder
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Cylinder pressure sensor glow plugSensor wiringEngine control module
What each part of P039C means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 3 | Area: ignition system or misfire. |
| 9C | The exact fault in that area. |
What to do next
- Note P039C and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P039C
What does P039C mean?
P039C means "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. It is a generic code, so it means the same on every make.
Is it safe to drive with P039C?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P039C fail an emissions test?
Yes, it can. P039C is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P039C to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P039C clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.