P2308: Cylinder 3 ion-sensor input error and the P0303 decision

P2308 is commonly labeled ignition coil C secondary circuit. Mazda's SKYACTIV-G page defines its monitored fault as an error in the number 3 ion-sensor signal reaching the PCM.

Why the misfire check comes first

The monitor applies after engine starting when engine speed is no more than 2,000 rpm. The manual lists cylinder 3 misfire as a potential cause and directs attention to P0303 before continuing with electrical diagnosis. This ordering preserves the distinction between abnormal combustion and a circuit that cannot report combustion correctly.

If the related misfire branch does not resolve the issue, the number 3 coil/ion-sensor connection and F8 15 A fuse are checked. The subsequent circuit branches cover missing supply, a grounded supply, an open ground, and signal faults to ground, to power or through an open conductor.

A useful repair conclusion

The range of possible circuit faults means that P2308 does not identify one failed wire or establish that the ignition coil itself is defective. The source reaches sensor inspection after evaluating those external paths and the PCM connection. It is important to retain the number 3 designation throughout the diagnosis rather than interpret the generic letter C as a universal physical location.

Mazda verifies the correction with an engine-running self-test and a pending-code inspection. A key-on result without engine operation does not reproduce the listed detection context. Any ignition testing should use the vehicle-specific professional precautions and procedure.

Documented application

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