Mechanical compression loss in cylinder 5 from worn or stuck piston rings or cylinder wall wear; Leaking or burnt exhaust or inlet valve, or poor valve seating, on cylinder 5; Cylinder head gasket leak affecting cylinder 5; Valve train fault such as a worn camshaft lobe, collapsed hydraulic lifter, or incorrect cam timing; Variable valve timing or valve lift system fault affecting cylinder 5 (depends on engine design); Faulty in-cylinder pressure sensor, or damaged wiring or connector to it, on engines that have one; Fuel or ignition fault on cylinder 5 causing weak combustion that the control unit reads as low pressure.

P03C2

Cylinder 5 Pressure Too 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
C2: FaultThe specific fault in that area

What it means

P03C2 means the engine control unit has judged that the pressure in cylinder 5 is lower than expected. How it reaches that depends on the engine. It may use an in-cylinder pressure sensor, or it may infer pressure from crankshaft speed variation or other strategies. The cylinder is not producing the combustion or compression the control unit expects.

Symptoms

Rough idle or a noticeable engine shake
Loss of power, especially under load
Engine management light on, possibly flashing if misfire is severe
Misfire codes stored alongside, such as for cylinder 5
Increased fuel consumption
Hard starting or uneven running when cold

Possible causes

Mechanical compression loss in cylinder 5 from worn or stuck piston rings or cylinder wall wearlikely
Leaking or burnt exhaust or inlet valve, or poor valve seating, on cylinder 5likely
Cylinder head gasket leak affecting cylinder 5possible
Valve train fault such as a worn camshaft lobe, collapsed hydraulic lifter, or incorrect cam timingpossible
Variable valve timing or valve lift system fault affecting cylinder 5 (depends on engine design)possible
Faulty in-cylinder pressure sensor, or damaged wiring or connector to it, on engines that have onerare
Fuel or ignition fault on cylinder 5 causing weak combustion that the control unit reads as low pressurepossible

How to diagnose

Read all stored and pending codes and the freeze frame data, noting load, engine speed and temperature when the fault set, and look for related misfire codes
Check for technical service bulletins or known issues for the specific engine, as the way this code is set varies between models
Inspect the wiring and connectors for cylinder 5 related sensors, injector and ignition coil for damage, corrosion or oil contamination
Check the ignition coil, spark plug and injector on cylinder 5, and swap with another cylinder to see if the fault follows
Check engine oil and coolant levels and look for signs of coolant loss, oil contamination or white exhaust smoke
Carry out a compression test on all cylinders and compare cylinder 5 with the others and with the manufacturer's specification
If compression is low, perform a cylinder leak-down test to locate the leak (rings, valves or head gasket)
Inspect valve timing, camshaft and lifter condition, and use a borescope on cylinder 5 if needed

How to fix

Repair or replace the faulty ignition, fuel or wiring components found on cylinder 5
Replace the faulty in-cylinder pressure sensor or repair its wiring where fitted
Repair or replace the head gasket, and check the head for flatness and cracks
Repair or replace leaking valves, seats, worn camshaft or lifters as found
Correct cam timing or repair the variable valve timing system fault
Carry out an engine overhaul on cylinder 5 for worn rings or bore, then clear the code and confirm it does not return

Can I keep driving?

Medium severity

Short, gentle journeys to a workshop are usually possible, but have it checked soon as a genuine compression loss can worsen and damage the catalytic converter or engine. If the engine management light flashes or the engine runs very roughly, stop driving and get it recovered.

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