What it means
The closed-loop combustion controller reports that combustion performance on cylinder 5 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 combustion controller uses the cylinder 5 pressure trace to judge combustion quality, including how much pressure the firing produces and when it develops. P03C6 sets when cylinder 5's calculated performance falls outside its calibrated target, which may reflect injector flow drift or a problem with pressure data or its communication path.
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
- Confirm P03C6 and capture freeze-frame data, then check for related cylinder 5 misfire, injector, pressure-sensor, or communication codes.
- Compare cylinder 5 pressure and combustion-performance live data with the other cylinders to see whether the issue is isolated.
- Inspect the cylinder 5 sensor and injector connectors and harnesses for damage, poor contact, or routing problems.
- Check cylinder 5 injector operation and flow using the manufacturer's diagnostic procedure.
- If the injector and sensor circuits test correctly, investigate combustion-control data routing and module communication using vehicle-specific diagnostics.
Common fixes
- Repair a loose or damaged cylinder 5 sensor or injector connection.
- Correct a confirmed cylinder 5 injector flow problem.
- Repair damaged wiring or a verified communication fault disrupting cylinder 5 pressure data.
- Replace a confirmed faulty pressure sensor or control component only after its circuit and data path are checked.
Can I keep driving?
Limit driving if the engine is rough, hesitates, or has reduced power, and avoid heavy loads until the cause is known. Poor cylinder 5 combustion can raise emissions and may worsen into a misfire.
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.