Fuel injector fault on cylinder 5 (leaking, sticking or poor spray pattern); Ignition fault on cylinder 5 on petrol engines (worn spark plug, weak coil, damaged lead or connector); Fault in the cylinder 5 pressure sensor or its wiring and connector, where fitted; Mechanical problem in cylinder 5 such as low compression, a leaking valve, worn piston rings or head gasket leak; Air leak, carbon build-up or restriction affecting the cylinder 5 intake port; Poor quality or contaminated fuel causing uneven combustion; Internal control unit fault.

P03C5

Cylinder 5 Pressure Variation High.

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
C5: FaultThe specific fault in that area

What it means

P03C5 means the engine control unit has seen the combustion pressure in cylinder 5 vary too much, either from cycle to cycle or compared with the other cylinders. It is a cylinder-specific code. The way pressure is measured depends on the engine: some have in-cylinder pressure sensors, while others estimate it from crank speed or similar signals. Exact thresholds and strategy vary by manufacturer and engine, so check the model-specific data.

Symptoms

Engine warning light on
Rough idle or noticeable vibration
Misfire or hesitation under load
Reduced power or the car going into limp mode
Increased fuel consumption
Harsh or uneven running, especially from cold

Possible causes

Fuel injector fault on cylinder 5 (leaking, sticking or poor spray pattern)likely
Ignition fault on cylinder 5 on petrol engines (worn spark plug, weak coil, damaged lead or connector)likely
Fault in the cylinder 5 pressure sensor or its wiring and connector, where fittedpossible
Mechanical problem in cylinder 5 such as low compression, a leaking valve, worn piston rings or head gasket leakpossible
Air leak, carbon build-up or restriction affecting the cylinder 5 intake portpossible
Poor quality or contaminated fuel causing uneven combustionrare
Internal control unit faultrare

How to diagnose

Read all stored and pending codes and note any misfire, injector or sensor codes alongside P03C5.
Review freeze frame data for engine speed, load and temperature when the fault set, and check live misfire counts for cylinder 5.
Visually inspect the cylinder 5 wiring, connectors and earths for damage, corrosion, oil or water ingress.
Check the cylinder 5 spark plug and coil (petrol) or injector electrical supply and resistance against the manufacturer's specification.
Swap the cylinder 5 coil or injector with a neighbouring cylinder, if the engine design allows, and see whether the fault follows.
Check for intake leaks and look at the live data for fuel trims and injector correction values on cylinder 5.
Carry out a compression or leak-down test on cylinder 5 to rule out mechanical faults.
Check the cylinder pressure sensor signal against the manufacturer's data, where the engine has one.

How to fix

Replace or properly clean and test the cylinder 5 fuel injector if it is faulty.
Replace the spark plug and ignition coil on cylinder 5, or on all cylinders if they are worn.
Repair damaged wiring or connectors, or replace the cylinder pressure sensor if it is faulty.
Repair the underlying mechanical fault, such as valve, piston ring or head gasket work, once testing confirms it.
Fix any intake leaks and clean carbon deposits from the intake tract using the manufacturer's recommended procedure.
Drain and replace contaminated fuel, then clear the code and road test to confirm the repair.

Can I keep driving?

Medium severity

Short, gentle journeys are usually possible, but get it checked soon because continued uneven combustion can damage the engine or catalytic converter. Avoid heavy loads and stop driving if the engine runs very roughly or the warning light flashes.

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