Ignition or injection fault on cylinder 5, such as a worn spark plug, failing coil or leaking or clogged injector; Loss of compression in cylinder 5 from worn rings, burnt or leaking valve, or head gasket leak; Air or vacuum leak near cylinder 5, such as a split intake runner or leaking manifold seal; Faulty cylinder pressure sensor or damaged wiring and connector, on engines that have one; Carbon build-up on valves or in the combustion chamber affecting cylinder 5 combustion; Valve timing fault or a worn camshaft lobe or lifter affecting cylinder 5; Faulty engine control unit or outdated software.

P03C4

Cylinder 5 Pressure Variation Low.

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
C4: FaultThe specific fault in that area

What it means

P03C4 means the engine control unit has detected that the combustion pressure in cylinder 5 is varying lower than expected from cycle to cycle or against the other cylinders. Depending on the engine, this is judged either from an in-cylinder pressure sensor signal or from calculated values such as crankshaft speed. Exact detection and thresholds vary by model and engine, so check the manufacturer's data for the vehicle.

Symptoms

Engine management light on
Rough idle or uneven running
Hesitation or loss of power under load
Misfire felt or heard, possibly with a flashing warning light
Increased fuel consumption
Hard starting or rough running when cold

Possible causes

Ignition or injection fault on cylinder 5, such as a worn spark plug, failing coil or leaking or clogged injectorlikely
Loss of compression in cylinder 5 from worn rings, burnt or leaking valve, or head gasket leakpossible
Air or vacuum leak near cylinder 5, such as a split intake runner or leaking manifold sealpossible
Faulty cylinder pressure sensor or damaged wiring and connector, on engines that have onepossible
Carbon build-up on valves or in the combustion chamber affecting cylinder 5 combustionpossible
Valve timing fault or a worn camshaft lobe or lifter affecting cylinder 5rare
Faulty engine control unit or outdated softwarerare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting load, rpm and temperature when the fault set, and look for related misfire, injector, fuel trim or pressure sensor codes.
Check live data for cylinder 5 misfire counts, crankshaft speed variation and fuel trims, and compare with the other cylinders.
Visually inspect the wiring and connectors for cylinder 5 (coil, injector and pressure sensor if fitted) for damage, corrosion, oil ingress or chafing.
Inspect the spark plug in cylinder 5 and compare its condition with the others.
Swap the coil with another cylinder, if the engine allows, and see whether any misfire follows it.
Check the intake system for leaks and carry out a smoke test if needed.
Carry out a compression test or leak-down test on cylinder 5 and compare it with the other cylinders.
Inspect the cylinder with a borescope for valve, piston or gasket damage, and check valve timing if compression is low.

How to fix

Replace the faulty spark plug, ignition coil or injector on cylinder 5, and clean the injector if the manufacturer permits.
Repair or replace damaged wiring, connectors or the cylinder pressure sensor where the circuit or sensor is at fault.
Repair any intake or vacuum leak found.
Carry out the necessary engine repair for a compression fault, such as valve, piston ring or head gasket work.
Correct the valve timing or replace worn valvetrain parts if they are the cause.
Update the engine control unit software only if the manufacturer lists an outdated calibration as a known cause for this code.

Can I keep driving?

Medium severity

Short, gentle journeys are possible, but get it checked soon because a continuing cylinder 5 fault can damage the engine or the catalytic converter. Stop driving if the warning light flashes, the engine runs very roughly or you hear knocking.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.