P039D code meaning
Cylinder 1 Pressure Variation High
Powertrain system Generic code P03xx codes Check engine light
What P039D 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 P039D as 'pressure variation too high' 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 large a swing points to a noisy or overshooting signal or implausible pressure peaks, for example from faulty sensor electronics, interference on the signal wire or, less often, a combustion anomaly such as a dripping injector. Usually there are no noticeable symptoms.
P039D 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 P039D
- Check engine light on
- Usually no noticeable symptoms
- Possibly rough running or combustion noise because control is off
Common causes of P039D
- Most likelySensor in the glow plug faulty (signal noise)
- PossibleLoose contact or interference on the signal wire
- Less likelyCombustion anomaly (dripping injector, harsh combustion)
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 P039D 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. |
| 9D | The exact fault in that area. |
What to do next
- Note P039D 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 P039D
What does P039D mean?
P039D means "Cylinder 1 Pressure Variation High". 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 P039D?
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 P039D fail an emissions test?
Yes, it can. P039D 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 P039D to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P039D 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.