Engine

P03B1 — Cylinder 3 Pressure Variation Too High

Severity: medium Drive with care, to a shop

What P03B1 means

The engine control module estimates cylinder pressure from crankshaft speed behavior, or reads it from an in-cylinder pressure sensor if the engine has one. P03B1 sets when cylinder 3 shows pressure changes from cycle to cycle that exceed the allowed limit. This is usually caused by unstable combustion, incorrect fuel delivery, or a loss of cylinder sealing.

Symptoms

  • Check engine light on
  • Rough idle
  • Misfires or hesitation on cylinder 3
  • Reduced power and sluggish acceleration
  • Higher fuel consumption
  • Possible fuel smell from the exhaust

Possible causes

  1. Worn spark plug or failing ignition coil on cylinder 3
  2. Faulty or clogged fuel injector on cylinder 3
  3. Low compression in cylinder 3 from valves, piston rings or head gasket
  4. Vacuum or intake air leak near cylinder 3
  5. Damaged wiring or connector at the coil, injector or cylinder pressure sensor
  6. Rarely, a faulty sensor or control module

How to diagnose

  1. Scan all codes and freeze frame data, and look for related misfire codes such as P0303
  2. Inspect the wiring and connectors for the cylinder 3 coil and injector
  3. Swap the cylinder 3 coil with a neighbor and see if the fault follows
  4. Remove the spark plug, check its condition and gap, and replace if worn
  5. Test the injector: resistance, operation, balance and flow
  6. Inspect the intake system and vacuum hoses for leaks
  7. Run a compression or leak-down test on cylinder 3
  8. Inspect the cylinder with a borescope if needed

Common fixes

  • Replace the spark plug and, if needed, the cylinder 3 ignition coil
  • Clean or replace the cylinder 3 fuel injector
  • Fix vacuum or intake leaks and replace damaged hoses or gaskets
  • Repair the damaged wiring or connector
  • Repair the engine internally: valves, rings, piston or head gasket

Questions and answers

What does P03B1 mean?

It means the engine computer detected too much pressure variation in cylinder 3. Ignition, fuel injector or poor compression in that cylinder are the most common causes.

Can I drive with code P03B1?

Only briefly and gently if the engine runs smoothly. If it misfires badly or the check engine light flashes, stop to avoid damaging the catalytic converter.

How do I start diagnosing P03B1?

Check the cylinder 3 spark plug and coil, and swap the coil with another cylinder to see if the problem follows. If it stays, test the injector and cylinder 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.