Engine

P03C2 — Cylinder 5 pressure too low

Severity: medium Drive with care, to a shop

What P03C2 means

The engine control unit, using a cylinder pressure sensor signal or crankshaft speed analysis, determined that pressure in cylinder 5 is lower than expected during compression or combustion. In a healthy engine every cylinder produces similar pressure, which keeps the engine smooth and the power steady. The cause can be a mechanical engine fault, a fuel or spark problem, or a sensor or wiring fault.

Symptoms

  • Check Engine light on
  • Rough idle or engine vibration
  • Weak acceleration and power loss
  • Higher fuel consumption
  • Stumbling or a feeling of misfires

Possible causes

  1. Low compression in cylinder 5: worn rings, leaking valves or a failed head gasket
  2. Faulty or clogged cylinder 5 fuel injector
  3. Failed spark plug or ignition coil on cylinder 5
  4. Vacuum leak upsetting the air/fuel mixture
  5. Faulty cylinder pressure sensor or damaged wiring giving false readings

How to diagnose

  1. Read all codes and freeze frame data and look for related misfire codes (P0305, etc.)
  2. Inspect the wiring and connectors for cylinder 5: injector, coil and pressure sensor
  3. Pull the cylinder 5 spark plug and inspect it, then swap the coil with a neighboring cylinder to see if the fault follows
  4. Test the injector's resistance and operation, and listen for clicking with a stethoscope
  5. Check the intake system for vacuum leaks (hoses, manifold gasket)
  6. Measure compression in cylinder 5 and compare it with the other cylinders
  7. If compression is low, run a cylinder leak-down test and inspect with a borescope
  8. Check the cylinder pressure sensor signal with a scope if the engine has one

Common fixes

  • Replace the spark plug and/or ignition coil on cylinder 5
  • Clean or replace the cylinder 5 fuel injector
  • Fix vacuum leaks and replace damaged intake parts
  • Repair the wiring or replace the faulty sensor
  • Repair the engine: rings, valves or head gasket

Questions and answers

What does code P03C2 mean?

It means the ECU sees too little pressure in cylinder 5. The cause may be a mechanical engine problem, fuel or spark trouble, or a sensor or wiring fault.

Can I drive with P03C2?

Only for a short distance and gently, to get to a shop. Driving long with a weak cylinder can damage the catalytic converter and the engine.

How do I tell a mechanical fault from an ignition fault?

Swap the coils and plugs between cylinders, then measure compression. If compression is normal, look at the ignition, injector or wiring.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.