What does it mean?
Combustion performance is the module's overall verdict on how well cylinder 6 is burning its charge, worked out from the pressure trace rather than from any single voltage. Using that signal the control module calculates where in the crank cycle the charge actually burns and how much work the cylinder produces, then compares both with the target derived from driver demand, fuelling and timing. When the deviation stays outside the allowed window even after the adaptive corrections have been applied, the module concludes the cylinder is genuinely underperforming and stores the code. The fault therefore points at the cylinder itself - fuelling, compression, valve control or ignition - far more often than at the sensor. Torque delivery becomes uneven, emissions rise and the MIL is set.
Symptoms
- MIL on, commonly together with a cylinder-specific misfire code
- Uneven idle with a recognisable beat and vibration at low engine speed
- Reduced power and poor pulling, especially uphill or fully loaded
- Higher fuel consumption and increased exhaust smoke or smell
- Harder starting and rough running until operating temperature is reached
- Failed emissions test even when the car still feels acceptable to drive
Likely causes
- Injector on cylinder 6 worn, coked or out of calibration, so fuel quantity and spray pattern no longer match the command
- Low compression from ring wear, valve leakage, incorrect valve clearance or a weakening head gasket
- Weak ignition on gasoline engines from a tired coil, an eroded plug or a leaking plug boot
- Glow plug failure on diesels, giving poor combustion until the engine is fully warm
- Camshaft timing drift or a variable valve timing fault altering the effective charge
- Fouled EGR or an intake runner heavily coated with carbon, reducing the air delivered to that cylinder
- Pressure sensor drifted or partially blocked, misreporting a cylinder that is actually healthy
How to fix it
- Read the freeze frame and note load and temperature, then graph pressure and contribution data for all cylinders side by side
- Run a relative compression test followed by a compression or leak-down test on cylinder 6 to settle the mechanical question early
- Check injector correction values and return quantity and, on gasoline engines, coil and spark plug condition
- Swap the injector or coil with an adjacent cylinder and see whether the poor combustion follows the component
- Inspect the glow plug circuit on diesels and check the intake runner and EGR passages for carbon restricting airflow to that cylinder
- Verify camshaft timing and variable valve timing operation from live data before turning to the sensor
- Confirm the pressure sensor is clean, correctly seated and torqued; repair the cause found, clear the code and re-check combustion balance on a road test