P2351 does not concern the sensor but the quality of combustion in cylinder 9, as judged from the pressure trace: the energy released per cycle, or its balance against the other cylinders, falls outside the permitted window. No manufacturer documents the code publicly, nor its cylinder-1 analogue P039E. CARB explains why such a check exists: engines fitted with combustion or combustion-quality sensors, such as homogeneous-charge compression-ignition designs, must catch misfire far more sensitively than others. Typical root causes are a coked or leaking injector, partial loss of compression, an air leak upstream or poor fuel. The engine runs unevenly, smoke rises, and the problem frequently shows up alongside a pressure too low/high code or a cylinder 9 misfire code.
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 9 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on
- Uneven running and vibration
- Increased smoke and fuel consumption
Most likely causes
- Injector of cylinder 9 coked, leaking or sticking
- Partial compression loss in cylinder 9
- Poor fuel quality or water in the fuel
- Pressure sensor distorting the evaluation
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 P2351 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- 51: Specific fault in that system
Frequently asked questions
Can I keep driving with code P2351?
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 P2351?
The most common causes are: injector of cylinder 9 coked, leaking or sticking; partial compression loss in cylinder 9; poor fuel quality or water in the fuel.
Related codes
- P234DCylinder 9 Pressure Too Low
- P234ECylinder 9 Pressure Too High
- P2349Cylinder 9 Pressure Sensor Circuit Range/Performance
- P0309Cylinder 9 Misfire Detected
- P039ECylinder 1 Combustion Performance
- P235BCylinder 10 Combustion Performance
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.