Engine

P03BB — Cylinder 4 Pressure Variation Too High

Severity: medium Drive with care, to a shop

What P03BB means

The engine control unit detected that combustion pressure in cylinder 4 varies from cycle to cycle more than allowed. This is measured with an in-cylinder pressure sensor or inferred from engine behavior. Normally each cylinder burns its mixture consistently with similar pressure. Large variation usually points to a misfire or unstable combustion. Exact definitions can differ between manufacturers.

Symptoms

  • Check Engine light on
  • Rough idle or shaking engine
  • Weak acceleration and loss of power
  • Worse fuel economy
  • Increased vibration or hesitation under load

Possible causes

  1. Worn or faulty spark plug in cylinder 4
  2. Failing ignition coil, or a bad coil wire or connector
  3. Clogged or faulty fuel injector on cylinder 4
  4. Intake or vacuum leak near cylinder 4
  5. Low compression from valve, ring or head gasket problems
  6. Faulty cylinder pressure sensor or its wiring

How to diagnose

  1. Scan all codes and freeze frame data, and look for related misfire codes such as P0304
  2. Inspect the cylinder 4 spark plug for wear, fouling, oil or cracks, and check the gap
  3. Swap the cylinder 4 coil with a neighbor and see whether the problem follows
  4. Check connectors and wiring for the coil, injector and pressure sensor for corrosion or damage
  5. Check the intake and vacuum hoses for leaks with a smoke machine or visual inspection
  6. Test injector operation and fuel pressure
  7. Run a compression or leak-down test on cylinder 4
  8. If mechanically sound, test the pressure sensor following the manufacturer's wiring diagram

Common fixes

  • Replace the spark plug and/or ignition coil on cylinder 4
  • Clean or replace the cylinder 4 fuel injector
  • Repair vacuum or intake leaks and replace damaged hoses or gaskets
  • Repair damaged wiring or connectors
  • Engine repair for low compression (valves, rings, head)

Questions and answers

What does P03BB mean?

It means combustion pressure in cylinder 4 is unstable and varies too much. The most common cause is a misfire from the plug, coil or injector.

Can I drive with P03BB?

Only briefly and gently, to get it checked. If the check engine light flashes, stop and arrange a tow to protect the catalytic converter.

Where should I start diagnosing P03BB?

Start with the cylinder 4 plug and coil: inspect the plug and swap coils between cylinders. If the problem stays, check the injector, vacuum leaks and 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.