Engine

P03A4 — Cylinder 2 Pressure Too Low

Severity: medium Drive with care, to a shop

What P03A4 means

The engine control unit monitors combustion or compression pressure in each cylinder, either with a cylinder pressure sensor or by estimating it from crankshaft speed variation. For cylinder 2 the value came in lower than expected, so the ECU stored P03A4. The cause can be weak ignition or fueling, a mechanical loss of cylinder sealing, or a sensor or wiring fault.

Symptoms

  • Check Engine light on
  • Rough idle or vibration
  • Reduced power under load
  • Hesitation or jerking when accelerating
  • Higher fuel consumption
  • Sometimes no noticeable symptoms

Possible causes

  1. Worn spark plug or weak ignition coil on cylinder 2
  2. Faulty fuel injector or fuel delivery problem at cylinder 2
  3. Vacuum or intake air leak near cylinder 2
  4. Mechanical loss of compression: valve, piston rings or head gasket
  5. Cylinder pressure sensor fault, or open, shorted or corroded wiring
  6. Crankshaft position sensor problem affecting the calculation

How to diagnose

  1. Read all codes and freeze frame data, and look for related misfire codes such as P0302.
  2. Inspect the cylinder 2 spark plug and coil, and swap the coil with a neighbor to see if the fault follows it.
  3. Test the cylinder 2 injector operation and resistance, and check fuel pressure.
  4. Inspect intake hoses and gaskets for leaks, using a smoke machine or fuel trim data.
  5. Check the pressure sensor connector and harness for power, ground and signal.
  6. Run a compression or leak-down test on cylinder 2.
  7. If valves or the head gasket are suspected, do a borescope inspection and test for combustion gases in the coolant.

Common fixes

  • Replace the spark plug or ignition coil on cylinder 2
  • Clean or replace the cylinder 2 fuel injector
  • Fix intake leaks and replace damaged hoses or gaskets
  • Repair the wiring or replace the cylinder pressure sensor
  • Repair the engine if the cause is mechanical: valves, rings or head gasket

Questions and answers

What does P03A4 mean?

The ECU found that pressure in cylinder 2 is lower than expected. It can come from ignition, fueling, an air leak, mechanical wear in the cylinder or a sensor fault.

Can I drive with P03A4?

Only for a short distance and under light load, to reach a shop. Driving long with a misfiring cylinder can damage the catalytic converter.

Where should I start diagnosing P03A4?

Start with the cylinder 2 spark plug and coil, since they are easy to test by swapping with a neighbor. If the code stays, check the injector, look for air leaks 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.