Engine

P03C4 — Cylinder 5 Pressure Variation Low

Severity: medium Drive with care, to a shop

What P03C4 means

The engine control unit monitors cylinder pressure, either with in-cylinder pressure sensors or indirectly through crankshaft speed changes, and compares each combustion cycle. P03C4 means pressure variation in cylinder 5 is lower than expected. This usually points to weak combustion caused by poor spark, insufficient fuel or low compression. A faulty sensor or wiring problem is also possible.

Symptoms

  • Check Engine light on
  • Rough or uneven idle
  • Hesitation or reduced power
  • Increased fuel consumption
  • Noticeable engine vibration
  • Hard starting

Possible causes

  1. Worn spark plug or failing ignition coil on cylinder 5
  2. Clogged or faulty fuel injector on cylinder 5
  3. Low compression in cylinder 5: rings, valves or head gasket
  4. Faulty in-cylinder pressure sensor or its wiring
  5. Valve timing or camshaft problem
  6. Vacuum leak near cylinder 5

How to diagnose

  1. Scan all codes and freeze-frame data, and look for misfire codes such as P0305
  2. Inspect the spark plug and coil on cylinder 5, then swap them with another cylinder to see if the fault follows
  3. Test the injector: resistance, operation and flow balance
  4. Inspect wiring and connectors for the coil, injector and pressure sensor, and check power and ground
  5. Check the intake for vacuum leaks
  6. Run a compression test on cylinder 5, and a leak-down test if needed
  7. Verify cam and crank timing
  8. Review per-cylinder live data with a scan tool

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 repair damaged wiring or connectors
  • Replace a faulty in-cylinder pressure sensor
  • Repair rings, valves or head gasket if compression is low
  • Correct valve timing

Questions and answers

What does P03C4 mean?

The ECU found that pressure in cylinder 5 varies less from cycle to cycle than it should. It most often points to weak combustion from spark, fuel or compression problems.

Can I drive with P03C4?

Briefly and gently, to reach a shop. If the engine shakes badly or the check engine light flashes, stop, since misfires can damage the catalytic converter and the cylinder.

How do I find the cause of P03C4 quickly?

Swap the coil and spark plug with a neighboring cylinder and see if the fault moves. If it stays, check the injector and test compression.

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.