P03BA: Cylinder 4 Pressure Variation Low

What it means

Cylinder-to-cylinder pressure variation on cylinder 4 is below the calibrated minimum — pressure stays unusually constant cycle-to-cycle, often a sign that the closed-loop combustion controller has lost authority or the sensor signal is filtered.

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 records pressure changes from one combustion cycle to the next, while the engine control module evaluates how consistently that cylinder is burning. This code sets when the calculated pressure variation for cylinder 4 is below the calibrated minimum, suggesting an unusually flat signal or a problem with signal processing or combustion control.

Common causes

Most likely first.

1. Combustion pressure sensor signal fault
2. Pressure signal filtering or processing fault
3. Combustion-control system fault

Symptoms

• Check-engine light
• Rough or uneven engine running
• Reduced engine power

How to diagnose it

1. Review freeze-frame data and live cylinder pressure readings to confirm the low variation is specific to cylinder 4 and note the operating conditions.
2. Compare the cylinder 4 pressure trace with traces from other cylinders to look for an unusually flat or filtered signal.
3. Inspect the cylinder 4 pressure sensor connector and wiring for damage, corrosion, poor connections, or routing that could affect the signal.
4. Check whether the scan tool reports related combustion-control, sensor, or module communication faults that could explain signal processing problems.
5. Test the cylinder 4 sensor output and its supply and ground circuits according to the manufacturer's procedures.
6. If the sensor circuit checks out, diagnose the combustion-control or signal-processing system using the manufacturer's service information.

Common fixes

• Repair a poor cylinder 4 pressure sensor connection or damaged wiring.
• Replace the cylinder 4 pressure sensor if testing confirms its signal is abnormally flat.
• Correct a confirmed signal filtering or processing fault in the combustion-control system.
• Repair a confirmed combustion-control system fault affecting cylinder 4.

Can I keep driving?

The code may be accompanied by rough running or reduced power because the module may not be assessing cylinder 4 combustion reliably. Avoid heavy loads if performance is affected, and arrange diagnosis soon to limit emissions and drivability problems.