P03C3: Cylinder 5 Pressure Too High

What it means

Cylinder 5 measured peak combustion pressure is above the calibrated maximum — typically pre-ignition, knock, over-fuelling, or runaway combustion.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

The cylinder 5 pressure sensor measures combustion pressure so the controller can detect abnormal firing in that cylinder. P03C3 sets when cylinder 5's peak pressure exceeds the calibrated limit, which can accompany knock, early combustion, or excessive fuel delivery.

Common causes

Most likely first.

1. Abnormally early combustion or engine knock
2. Excessive fuel delivery to cylinder 5
3. Incorrect ignition timing
4. Combustion-chamber deposits

Symptoms

• Check-engine light
• Engine knocking or pinging
• Rough running
• Reduced engine power

How to diagnose it

1. Treat the fault as urgent if the engine is knocking; record freeze-frame data and check for related knock, ignition, and fueling codes.
2. Review cylinder 5 pressure and knock-related live data to see whether the high reading occurs under load or at a particular engine speed.
3. Inspect the cylinder 5 pressure-sensor connector and wiring for a signal fault that could falsely report excessive pressure.
4. Check cylinder 5 injector operation and compare its delivery with the manufacturer's diagnostic procedure.
5. If the sensor and fuel delivery check out, investigate ignition timing where applicable and inspect for combustion-chamber deposits using vehicle-specific procedures.

Common fixes

• Repair a damaged cylinder 5 pressure-sensor connector or wiring if testing identifies a signal fault.
• Correct confirmed excessive fuel delivery from the cylinder 5 injector.
• Repair an ignition or timing fault identified by diagnosis, where applicable.
• Address combustion-chamber deposits or another confirmed cause of excessive cylinder 5 pressure.

Can I keep driving?

Avoid driving while the engine is knocking, running roughly, or in reduced-power mode; arrange prompt diagnosis and stop if the knocking is severe. Excessive cylinder 5 pressure can damage the piston, valves, or other engine parts.