P03B5: Cylinder 4 pressure circuit below its valid electrical range

P03B5 identifies low input from cylinder pressure sensor number 4 in Mazda's SKYACTIV-X engine. The documented electrical threshold is 0.29 V or below for five seconds.

Preserve the multi-cylinder pattern

Mazda checks for P0397, P03A1 and P03AB before treating number 4 as an isolated fault. A companion low-input code moves diagnosis toward the PCM connection. Erasing the whole code set before recording it can lose this distinction and encourage unnecessary individual sensor replacements.

The isolated route examines the number 4 connector and sensor resistance. The remaining checks cover grounding of the supply or signal, a short between signal and sensor ground, and open supply or signal circuits. An internal sensor malfunction remains possible, but the source supplies several external explanations for the same voltage.

What the code does and does not establish

P03B5 reports a circuit-limit problem; it does not establish insufficient compression in cylinder 4. Mazda separately includes a functional sensor check after the relevant electrical investigation. A correct connector fit alone does not prove valid pressure feedback.

The fail-safe inhibits EGR and SPCCI, stops combustion feedback and retards timing. Verification requires the prescribed key-on or running self-test and a check for recurring pending faults. Match the cylinder and wiring information to the actual engine before any professional sensor testing.

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.