P03B2: Cylinder 3 combustion signal reliability fault

P03B2 concerns cylinder 3 combustion information in the documented SKYACTIV-X system. Mazda labels the condition abnormal combustion and explains that the PCM cannot reliably determine combustion status from the pressure-sensor signal.

Electrical and physical possibilities

The monitor can respond to an unreliable calculated combustion value or an abnormal signal voltage that continues for the specified period. It is enabled only within the relevant control region, with no misfire, no fuel cut and specified pressure-sensor faults absent. These conditions distinguish it from both a straightforward misfire code and an electrical high/low code.

The cause list spans poor connections, sensor insulation or sensor failure, wiring faults, incorrect injection timing and injector malfunction. Compression loss and coolant entering a combustion chamber are also included. The fault name therefore does not decide whether the measurement system or the combustion process is responsible.

Evidence that changes the repair

Mazda first checks pressure data for connector sensitivity and then separates sensor-circuit and sensor testing from injector, compression and coolant-leak investigations. A confirmed unstable number 3 signal supports a different repair from normal sensor behavior with a verified mechanical fault.

The fail-safe disables SPCCI and EGR, stops combustion feedback and retards ignition. Repair confirmation requires the prescribed engine-running condition and a pending-code check. The manual's malformed related-code text is not reproduced as a diagnostic list; use the exact current service procedure for that branch.

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.