P03BB: Cylinder 4 Pressure Variation High

What it means

Cylinder-to-cylinder pressure variation on cylinder 4 is above the calibrated maximum — combustion is unstable cycle-to-cycle, typical of partial misfire or injector seat erosion.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The cylinder 4 pressure sensor captures pressure for each combustion cycle, and the engine control module uses those readings to assess combustion consistency. This code sets when pressure variation in cylinder 4 is greater than the calibrated maximum, which can happen with incomplete combustion, injector trouble, ignition faults where fitted, or low compression.

Common causes

Most likely first.

1. Misfire or incomplete combustion in cylinder 4
2. Fuel injector fault
3. Ignition fault, where applicable
4. Low compression

Symptoms

• Check-engine light
• Rough or uneven idle
• Engine misfire or hesitation
• Reduced engine power

How to diagnose it

1. Review freeze-frame data, cylinder 4 pressure variation, and misfire data to identify when the instability occurs and whether it is isolated to cylinder 4.
2. Inspect the cylinder 4 pressure sensor connector and harness for loose terminals, corrosion, chafing, or heat damage that could create erratic readings.
3. Check cylinder 4 fuel-injector operation using scan data and the manufacturer's specified tests.
4. Where the engine has spark ignition, inspect and test the cylinder 4 ignition components according to the manufacturer's procedures.
5. If sensor, fueling, and ignition checks do not explain the variation, measure cylinder 4 compression and compare it with the manufacturer's specification.
6. Use a leak-down test if compression is low to locate leakage through the valves, rings, or other sealing surfaces.

Common fixes

• Repair a loose or damaged cylinder 4 sensor connection or harness.
• Repair or replace a cylinder 4 injector that fails the specified test.
• Repair a confirmed cylinder 4 ignition fault where applicable.
• Repair the valve, ring, or other sealing problem identified by compression and leak-down tests.

Can I keep driving?

Rough idle, misfire, or hesitation can indicate unstable combustion in cylinder 4. Avoid hard acceleration, and stop driving if the engine misfires severely or loses substantial power because continued misfiring can damage the engine or emissions system.