What it means
The closed-loop combustion controller reports that combustion performance on cylinder 4 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 engine's closed-loop combustion controller uses the cylinder 4 pressure trace to evaluate combustion quality, including measures of pressure and combustion timing. This code sets when the calculated performance for cylinder 4 falls outside the module's calibrated range, so the pressure signal, fuel delivery, or combustion control may be at fault.
Common causes
Most likely first.
- Injector flow rate drift
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Check-engine light
- Rough or uneven engine running
- Misfire or hesitation
- Reduced engine power
How to diagnose it
- Review freeze-frame data and live cylinder 4 combustion and pressure readings to see which conditions coincide with the performance fault.
- Compare cylinder 4 readings with the other cylinders and check for related misfire, injector, sensor, or communication codes.
- Inspect cylinder 4 injector wiring and pressure sensor connections for damage, corrosion, or a poor fit.
- Test cylinder 4 injector operation and flow using the manufacturer's specified procedure.
- Check the pressure sensor signal and its supply and ground circuits against the manufacturer's specification.
- If sensor and injector tests pass, diagnose combustion-control communication and processing faults using the vehicle's wiring diagram and service information.
Common fixes
- Repair a loose, corroded, or damaged cylinder 4 pressure sensor or injector connection.
- Repair or replace a cylinder 4 injector that fails the specified flow or operation test.
- Replace a cylinder 4 pressure sensor confirmed to be reporting inaccurate data.
- Repair a confirmed combustion-control communication or processing fault.
Can I keep driving?
Rough running, hesitation, or reduced power can mean cylinder 4 combustion is not being controlled or measured correctly. Avoid heavy loads, and stop driving if misfiring becomes severe because continued operation can worsen engine and emissions-system damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.