P03B4: Cylinder 4 pressure function and compression plausibility

P03B4 is the SKYACTIV-X cylinder 4 pressure-sensor function code. Mazda associates it with a pressure signal that does not agree with the engine's expected pressure magnitude or timing.

The model provides context, not a component verdict

The PCM estimates pressure near top dead center using intake-pressure information and intake-valve timing. It also checks whether the pressure peak is late and whether the pressure difference around top dead center is excessive. These separate evaluations can all produce this DTC.

The published preconditions include fuel cut, closed EGR, inhibited purge and valid pressure, crankshaft and camshaft signals. Electrical high/low pressure-sensor faults inhibit the function check. Thus, a plausible-looking voltage alone does not prove the sensor is accurate enough for combustion control.

Follow the possible physical causes

Mazda checks related fuel-system faults, connections and the pressure-sensor circuit, then the sensor and engine compression. Incorrect injection timing or poor compression can change measured pressure without an open sensor wire. That is why cylinder 4 identification must be combined with actual test results.

The fail-safe removes EGR and SPCCI control, stops combustion feedback and retards ignition. After a supported repair, verification must exercise the relevant deceleration monitor and check for pending recurrence. The page does not provide a universal pressure-difference limit, and it does not establish that the engine needs overhaul merely because P03B4 is present.

Documented application

Mazda3 workshop manual 01/2019 onward, d9n1-1a-22d_ver10; PCM (SKYACTIV-X). Verify exact vehicle specification; this is a Mazda application example. Mazda factory diagnostic page, mirrored by CHARM.