Engine

P03B2 — Cylinder 3 Combustion Performance Fault

Severity: medium Drive with care, to a shop

What P03B2 means

The engine control unit detected that combustion in cylinder 3 is weaker than expected, which can mean a misfire or a poor power stroke. Normally each cylinder gets the right air-fuel mix, a strong spark and good compression, and the ECU monitors crankshaft speed to confirm it. The exact detection logic varies by manufacturer, so freeze frame and misfire data are useful.

Symptoms

  • Check engine light on or flashing
  • Rough or uneven idle
  • Hesitation or stumbling when accelerating
  • Loss of power
  • Poor fuel economy
  • Fuel smell from the exhaust

Possible causes

  1. Worn or faulty spark plug in cylinder 3
  2. Failing ignition coil or connector for cylinder 3
  3. Clogged or leaking fuel injector on cylinder 3
  4. Vacuum leak near cylinder 3
  5. Low compression from valve, piston ring or head gasket problems
  6. Damaged wiring or poor connection at the coil or injector

How to diagnose

  1. Read all codes and freeze frame data and look for other misfire or fuel trim codes
  2. Inspect the cylinder 3 spark plug for wear, gap, deposits, oil or fuel fouling
  3. Swap the cylinder 3 coil with a neighbor, clear the code and see if the fault follows
  4. Check coil and injector connectors and wiring for corrosion or damage
  5. Test the injector's resistance and operation, or listen to it with a stethoscope
  6. Inspect intake hoses and gaskets for vacuum leaks, using a smoke machine if needed
  7. Run a compression or cylinder leak-down test
  8. Check fuel pressure and fuel trim data

Common fixes

  • Replace the spark plugs, ideally as a set
  • Replace the cylinder 3 ignition coil
  • Clean or replace the cylinder 3 fuel injector
  • Repair vacuum leaks and replace damaged hoses or gaskets
  • Repair wiring or the connector
  • Repair the engine (valves, rings, head gasket) if compression is low

Questions and answers

What does P03B2 mean?

It means the ECU found a combustion performance problem in cylinder 3. The usual causes are a spark plug, coil or injector, and less often an internal engine fault.

Can I drive with P03B2?

Only carefully and for a short distance, such as to a shop. If the check engine light is flashing, do not drive because the catalytic converter can overheat.

Where should I start diagnosing P03B2?

Inspect the cylinder 3 spark plug and swap the coil with a neighboring cylinder. If the fault moves to that cylinder, the coil is bad.

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.