P03BC
Cylinder 4 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
- BC
- FaultThe specific fault in that area
What it means
P03BC is the SAE J2012 generic code for Cylinder 4 Combustion Performance. The engine control unit has judged that combustion in cylinder 4 is poor or inconsistent compared with the other cylinders. It usually works this out from crankshaft speed variation or similar data, and it may or may not be accompanied by a conventional misfire code. The exact detection method varies by manufacturer and engine.
Symptoms
- Engine management light on, possibly flashing under load
- Rough idle or vibration, especially when cold
- Hesitation, juddering or loss of power when accelerating
- Increased fuel consumption
- Smell of unburnt fuel from the exhaust
- Hard starting or occasional stalling
Possible causes
- Worn or fouled spark plug on cylinder 4 (petrol engines)likely
- Faulty ignition coil or coil connector on cylinder 4likely
- Fuel injector on cylinder 4 blocked, leaking, sticking or with a wiring faultlikely
- Vacuum or intake leak affecting cylinder 4, such as a worn manifold gasket or cracked hosepossible
- Low compression in cylinder 4 from a valve, piston ring or head gasket problempossible
- Valve train fault such as worn cam lobe, faulty variable valve timing component or incorrect timingrare
- Contaminated or poor quality fuel, or an engine control unit software issue that is a known fault for that enginerare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting engine load, temperature and speed when the fault set, and look for related misfire, injector or fuel trim codes.
- Inspect the cylinder 4 coil, injector and their connectors and wiring for damage, corrosion, oil contamination or loose pins.
- Check the spark plug for cylinder 4 for wear, fouling, oil or incorrect gap, and compare it with the other cylinders (petrol engines).
- Swap the cylinder 4 coil with another cylinder, clear the codes and retest to see whether the fault follows the coil. Repeat with the spark plug if needed.
- Check live data for fuel trims and per-cylinder misfire or roughness counters, and test the injector's resistance and operation as per the manufacturer's data.
- Inspect for intake leaks with a smoke test and check the manifold gasket and vacuum hoses.
- Carry out a compression test and, if the result is low, a leak-down test to find the cause.
- Check for any manufacturer technical bulletins or software updates relating to this code on the specific model and engine.
How to fix
- Replace the worn spark plug set, using the specified type and gap.
- Replace the faulty ignition coil, or repair its connector or wiring.
- Clean or replace the faulty fuel injector, or repair its wiring or connector.
- Repair intake leaks by replacing the gasket, seals or damaged hoses.
- Carry out the necessary engine repair for low compression, such as valve, piston ring or head gasket work, once the fault is confirmed.
- Where an outdated engine control unit calibration is a known cause for that engine, update it to the manufacturer's current version.
Can I keep driving?
Short, gentle driving is usually possible, but get it fixed soon because unburnt fuel can overheat and damage the catalytic converter. If the engine management light flashes, the engine runs very roughly or power is badly reduced, stop driving and arrange recovery.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.