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. P039E means the combustion quality in the cylinder, evaluated from the combustion pressure, is abnormal. No manufacturer document available to us describes the code; a GM patent shows the method: the ratio of measured to motoring pressure is turned into a measure of combustion energy, an inequality index against the neighbouring cylinders in firing order is formed, and a two-sided threshold detects partial combustion, misfire, injector and compression faults. CARB requires engines with a combustion or combustion quality sensor to detect misfire more sharply (5 % in 1,000 revolutions). Typical causes are a coked or leaking injector, compression loss or poor fuel; the engine runs roughly and smokes more.
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
- Rough running, vibration
- Loss of power, more smoke and consumption
Most likely causes
- Injector coked, leaking or delivering the wrong quantity
- Compression loss (valve, rings)
- Partial combustion or misfire (fuel quality, EGR distribution)
- Pressure sensor drifting
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 P039E means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- 9E: Specific fault in that system
Frequently asked questions
Can I keep driving with code P039E?
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 P039E?
The most common causes are: injector coked, leaking or delivering the wrong quantity; compression loss (valve, rings); partial combustion or misfire (fuel quality, EGR distribution).
Related codes
- P039ACylinder 1 Pressure Too Low
- P039BCylinder 1 Pressure Too High
- P0396Cylinder 1 Pressure Sensor Circuit Range/Performance
- P0301Cylinder 1 Misfire Detected
- P0263Cylinder 1 Contribution/Balance
Sources
- patents.google.com: US8229655
- law.cornell.edu: 13 CCR 1968.2
- borgwarner.com: pressure sensor glow plugs
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.