P03C6
Cylinder 5 Combustion Performance.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 3
- AreaIgnition system or misfire
- C6
- FaultThe specific fault in that area
What it means
P03C6 means the engine control unit has judged that combustion in cylinder 5 is not performing as expected. How it measures this depends on the vehicle. It may use crankshaft speed variation, in-cylinder pressure or ion-current sensing, or a comparison against the other cylinders. The fault usually appears alongside misfire or fuel trim codes.
Symptoms
- Engine management light on, possibly flashing under load
- Rough idle or noticeable vibration
- Hesitation, jerking or loss of power when accelerating
- Poor fuel economy
- Hard starting or an uneven cold start
- Smell of unburnt fuel or a rougher exhaust note
Possible causes
- Worn, fouled or incorrectly gapped spark plug on cylinder 5 (petrol engines)likely
- Faulty ignition coil or coil connector on cylinder 5likely
- Faulty, blocked or leaking fuel injector on cylinder 5likely
- Vacuum or intake leak affecting cylinder 5, or a damaged intake valve seal or carbon build-up on the intake valvespossible
- Low compression in cylinder 5 from a burnt valve, worn rings or a head gasket leakpossible
- Wiring or connector fault in the injector, coil or in-cylinder sensor circuit for cylinder 5possible
- Faulty crankshaft position sensor or in-cylinder pressure sensor giving false readings, or an outdated control unit calibration where the manufacturer has issued a fixrare
How to diagnose
- Read all stored and pending codes and note the freeze frame data (load, rpm, coolant temperature, fuel trims). Look for related misfire codes on cylinder 5 or other cylinders.
- Check the cylinder numbering and firing order for your specific engine so you are testing the right cylinder.
- Inspect the cylinder 5 coil, injector and sensor connectors and wiring for damage, corrosion, oil contamination or poor pin tension.
- Remove and inspect the cylinder 5 spark plug (if petrol) and compare it with the others for wear, deposits or incorrect gap.
- Swap the cylinder 5 coil with another cylinder and see whether the fault follows it. Do the same with the injector where the engine allows.
- Check for intake leaks with a visual inspection and smoke test, and review fuel trims for signs of a lean condition.
- Test injector operation and resistance, then check the injector's electrical supply and control signal.
- Carry out a compression or leak-down test on cylinder 5. Consult model-specific data for any in-cylinder sensor tests.
How to fix
- Replace the spark plugs, as a set where appropriate, with the correct specification.
- Replace the faulty ignition coil on cylinder 5.
- Clean or replace the faulty fuel injector on cylinder 5, depending on the test results.
- Repair damaged wiring or connectors in the affected circuit.
- Repair intake leaks, or clean carbon deposits from the intake valves using a method suited to your engine.
- Carry out the necessary mechanical repair if compression is low (valves, rings or head gasket). Apply a manufacturer-issued calibration update only if one exists for this fault.
Can I keep driving?
Medium severity
Short, gentle journeys to a workshop are usually acceptable, but fix it soon because unburnt fuel can overheat and damage the catalytic converter. If the warning light flashes or the engine runs very roughly, stop driving and have it checked.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.