Engine

P03BC — Cylinder 4 Combustion Performance Fault

Severity: medium Drive with care, to a shop

What P03BC means

The engine control module uses crankshaft speed and other signals to judge how well each cylinder burns its mixture. P03BC is set when cylinder 4 is not combusting as well as expected, usually because of a misfire or unstable cylinder output. Good combustion needs a strong spark, the right amount of fuel, enough air and healthy compression.

Symptoms

  • Rough or shaky idle
  • Check engine light on or flashing
  • Hesitation or stumbling during acceleration
  • Loss of power
  • Higher fuel consumption
  • Fuel smell from the exhaust

Possible causes

  1. Worn or faulty spark plug in cylinder 4
  2. Failed ignition coil on cylinder 4 or a bad coil connector
  3. Clogged or faulty fuel injector on cylinder 4
  4. Vacuum or intake air leak near cylinder 4
  5. Damaged wiring or poor connection at the injector or coil
  6. Low compression in the cylinder (valve, rings or head gasket)
  7. Faulty oxygen sensor or another sensor affecting the mixture

How to diagnose

  1. Scan all codes and freeze frame data, and look for related misfire codes such as P0304
  2. Inspect the connectors and wiring at the cylinder 4 coil and injector
  3. Remove the cylinder 4 spark plug and check wear, deposits, gap and any oil or fuel fouling
  4. Swap the coil with a neighboring cylinder and see whether the fault follows
  5. Check for vacuum leaks by inspecting hoses and ducts or using a smoke machine
  6. Test the injector's resistance and listen for its operation
  7. Run a compression or cylinder leak-down test
  8. Review fuel trims and oxygen sensor readings

Common fixes

  • Replace spark plugs, starting with cylinder 4
  • Replace the cylinder 4 ignition coil
  • Clean or replace the cylinder 4 fuel injector
  • Fix vacuum leaks and replace damaged hoses or gaskets
  • Repair wiring and connectors
  • Perform engine repairs if compression is low

Questions and answers

What does P03BC mean?

The ECM found poor combustion in cylinder 4, most often caused by a misfire. Ignition, fuel delivery and air supply should all be checked.

Can I drive with P03BC?

Only briefly and under light load. Continued misfires can damage the catalytic converter, and a flashing check engine light means you should avoid driving.

What should I check first?

Start with the spark plug and ignition coil on cylinder 4, as they are the most common causes. Swapping the coil to another cylinder quickly shows whether it is at fault.

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.