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. P039B means the pressure measured or calculated for the cylinder is higher than expected. It is documented by code at Audi, surprisingly on a petrol engine: on the 2015 S3 (2.0 TFSI, engine CYFB) the four 'pressure too high' codes came on with the EPC lamp, usually together with misfire codes P0300–P0304 and the manufacturer code for hiding a cylinder; the cause was a software fault that mis-detected misfire, and a software update fixed it. How Audi determines the pressure, the bulletin does not say. On diesels with pressure-sensing glow plugs GM patents describe a peak pressure above its limit – too much or too early injection, or a sensor drifting upwards. If the cylinder is shut off, power drops noticeably.
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
- EPC or check engine light on
- Misfire codes, possibly a cylinder shut off with noticeable loss of power
- Rough running
Most likely causes
- Engine module software (Audi S3 2015, engine CYFB)
- Injector delivering too much or dripping
- Sensor drifting upwards (diesel pressure-sensing glow plug)
- Knock or pre-ignition on a petrol engine
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P039B means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- 9B: Specific fault in that system
Frequently asked questions
Can I keep driving with code P039B?
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 P039B?
The most common causes are: engine module software (Audi S3 2015, engine CYFB); injector delivering too much or dripping; sensor drifting upwards (diesel pressure-sensing glow plug).
Related codes
- P039ACylinder 1 Pressure Too Low
- P0396Cylinder 1 Pressure Sensor Circuit Range/Performance
- P039ECylinder 1 Combustion Performance
- P0301Cylinder 1 Misfire Detected
- P03A5Cylinder 2 Pressure Too High
- P03AFCylinder 3 Pressure Too High
Sources
- static.nhtsa.gov: MC 10153584 9999
- static.nhtsa.gov: MC 10128395 9999
- patents.google.com: US7726281
- law.cornell.edu: 13 CCR 1968.2
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.