P039E: Cylinder 1 combustion information is unreliable

P039E is the cylinder 1 combustion-status fault in Mazda's SKYACTIV-X pressure-sensing system. The factory wording is abnormal combustion, but its explanation emphasizes whether the pressure signal can be trusted to determine combustion status.

Why it is not simply a misfire code

The monitor requires no misfire and no active fuel cut, and specified pressure-sensor function or circuit faults must be absent. It then identifies an unreliable combustion value or abnormal pressure-sensor signal that persists for the prescribed period. The page does not publish that duration.

Poor sensor insulation or another signal defect can prevent the PCM from interpreting combustion correctly. Conversely, incorrect injection timing, an injector problem, inadequate compression or coolant entering the combustion chamber can alter the physical event being measured.

Distinguish measurement from combustion

Mazda evaluates pressure data while checking connections, then follows separate sensor-circuit, sensor, injector, compression and coolant-leakage branches. Those branches explain why the code is not sufficient to order a cylinder pressure sensor or an injector.

The controller inhibits EGR and SPCCI, stops combustion feedback and retards ignition. Professional verification must reproduce the specified running condition after the relevant fault is repaired, then check pending records. Preserve cylinder 1 pressure and related engine data before clearing memory. The source contains malformed related-misfire code text, so this article relies on its diagnostic headings and supported system explanation rather than repeating that list.

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.