P2351 Cylinder 9 Combustion Performance

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.

Likely causes

Symptoms

• Check engine light on
• Uneven running and vibration
• Increased smoke and fuel consumption

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, borgwarner.com.