P039A code meaning
Cylinder 1 Pressure Too Low
Powertrain system Generic code P03xx codes Check engine light
What P039A 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. P039A means the pressure measured in the cylinder is lower than expected while the sensor itself is electrically fine. No manufacturer document available to us describes the code; GM patents show how such an evaluation works: peak and minimum pressure are compared with limits and with a motoring pressure calculated from manifold pressure and compression ratio, and the ratio of measured to motoring pressure is compared between cylinders. A low value can mean weak combustion or compression – leaking valve, worn rings, leaking glow plug seat, an injector delivering too little – or a sensor drifting downwards. The usual consequence is rough running and more smoke.
P039A at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyHard
- Typical labor1 to 6 hours
- Clears after repairAfter a drive cycle
Symptoms of P039A
- Check engine light on
- Lack of power, rough running, possibly misfire on this cylinder
- More smoke
Common causes of P039A
- PossibleCompression loss (valve, piston rings, leaking glow plug seat)
- PossibleInjector delivering too little or coked
- PossibleSensor drifting downwards (deposits)
- Less likelyMisfire or partial 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 moduleFuel injector
What each part of P039A 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. |
| 9A | The exact fault in that area. |
What to do next
- Note P039A 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 P039A
What does P039A mean?
P039A means "Cylinder 1 Pressure Too 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 P039A?
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 P039A fail an emissions test?
Yes, it can. P039A 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 P039A to fix?
Difficulty: hard. A typical repair takes 1 to 6 hours of work. This is a job for a workshop.
Will P039A 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.