P2351: Cylinder 9 Combustion Performance

What it means

The closed-loop combustion controller reports that combustion performance on cylinder 9 is below the calibrated quality metric. Computed from the cylinder pressure trace — IMEP (indicated mean effective pressure), CA50 (50% mass burnt angle) or peak pressure deviates from the model.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The closed-loop combustion controller evaluates cylinder 9 using pressure-derived measures such as indicated mean effective pressure, combustion timing, or peak pressure. This code sets when cylinder 9’s measured combustion performance falls outside the controller’s calibrated model, even if the pressure sensor circuit itself is not reporting an electrical fault.

Common causes

Most likely first.

1. Mean effective pressure outside model
2. Injector flow has drifted
3. Gateway module routing CAN messages incorrectly

Symptoms

• Check-engine light
• Rough running or misfire
• Reduced engine power
• Poor acceleration

How to diagnose it

1. Save freeze-frame data and review live cylinder 9 combustion-quality and misfire information to identify the operating conditions that trigger the code.
2. Inspect cylinder 9’s injector wiring and connector, then check for signs of an intermittent connection.
3. Compare cylinder 9’s pressure-derived performance measures with the other cylinders under matching operating conditions.
4. Test cylinder 9 injector flow or contribution using the manufacturer’s procedure to check for drift from expected delivery.
5. If the cylinder and injector checks pass, have the relevant CAN data routing and gateway communication checked for errors.

Common fixes

• Repair a poor connection or damaged wiring at cylinder 9’s injector or related sensor.
• Correct a cylinder 9 injector flow fault confirmed by testing.
• Repair a verified CAN communication or gateway routing fault.
• Address any mechanical condition confirmed to be affecting cylinder 9 combustion.

Can I keep driving?

If the engine runs smoothly and power is normal, limit hard acceleration and arrange diagnosis soon. If it misfires or loses power, avoid continued driving because poor combustion can increase emissions and cause further damage.