Engine

P03B8 — Cylinder 4 Pressure Too Low

Severity: medium Drive with care, to a shop

What P03B8 means

The ECU detected, through crankshaft speed variation, sensor data or in some engines a cylinder pressure sensor, that combustion or compression pressure in cylinder 4 is lower than expected. In a healthy engine each cylinder seals well and contributes about the same power. Low pressure points to a leak, poor cylinder filling or incomplete combustion in that cylinder.

Symptoms

  • Check Engine light on
  • Rough idle or engine vibration
  • Loss of power or hesitation
  • Misfire on cylinder 4
  • Worse fuel economy
  • Higher exhaust emissions

Possible causes

  1. Low compression in cylinder 4: worn or stuck piston rings, burnt valve, damaged piston
  2. Head gasket leak at cylinder 4
  3. Faulty or clogged cylinder 4 fuel injector
  4. Ignition fault: cylinder 4 spark plug or coil
  5. Vacuum leak near the cylinder 4 intake
  6. Sensor or wiring fault causing the ECU to receive incorrect data

How to diagnose

  1. Read all codes and freeze frame data, and check for misfire codes such as P0304
  2. Inspect the cylinder 4 spark plug and coil, then swap the coil with a neighbor to see if the fault follows
  3. Test the cylinder 4 injector: listen for clicking, measure resistance, check spray or balance
  4. Inspect the intake for vacuum leaks (smoke test, hoses, manifold gasket)
  5. Check the wiring harness and connectors for cylinder 4 for breaks, corrosion or shorts to ground
  6. Perform a compression test and compare cylinder 4 with the others
  7. If compression is low, run a leak-down test to locate the leak: valves, rings or head gasket
  8. If needed, check cylinder pressure with a scope or inspect the cylinder with a borescope

Common fixes

  • Replace the cylinder 4 spark plug and/or ignition coil
  • Clean or replace the cylinder 4 injector
  • Fix vacuum leaks, replace the intake manifold gasket or damaged hoses
  • Repair the wiring or replace the faulty sensor
  • Repair the engine: valves, piston rings or head gasket

Questions and answers

What does P03B8 mean?

The ECU found that pressure in cylinder 4 is lower than it should be. The cause can be ignition, fuel delivery or a mechanical engine problem.

Can I drive with P03B8?

Only for a short distance and under light load to reach a shop. If the engine runs rough or the Check Engine light flashes, stop driving to avoid damaging the catalytic converter.

What should I check first?

Start with the cylinder 4 spark plug and coil, then the injector and vacuum leaks. If all are fine, do a compression test.

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.