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.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Cylinder 1 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on
- Lack of power, rough running, possibly misfire on this cylinder
- More smoke
Most likely causes
- Compression loss (valve, piston rings, leaking glow plug seat)
- Injector delivering too little or coked
- Sensor drifting downwards (deposits)
- Misfire or partial combustion
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It affects emissions.
What each character of P039A means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- 9A: Specific fault in that system
Frequently asked questions
Can I keep driving with code P039A?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P039A?
The most common causes are: compression loss (valve, piston rings, leaking glow plug seat); injector delivering too little or coked; sensor drifting downwards (deposits).
Related codes
- P039BCylinder 1 Pressure Too High
- P0396Cylinder 1 Pressure Sensor Circuit Range/Performance
- P039ECylinder 1 Combustion Performance
- P0301Cylinder 1 Misfire Detected
- P03A4Cylinder 2 Pressure Too Low
- P03AECylinder 3 Pressure Too Low
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.