P0308: cylinder-8 misfire and completed test frames

P0308 identifies cylinder-8 misfire in a diagnostic that evaluates defined revolution frames. A cleared code or a zero counter immediately after restart is not equivalent to completion of those frames.

What the source counts

The startup path covers the first 1,000 crankshaft revolutions and lists total misfires above 120. Cylinder-8 attribution compares its count with ten percent of total engine misfires. The later emissions path again uses 1,000-revolution frames, with a four-frame counter.

The continuation adds a catalyst-damage branch based on weighted exhaust-bank counts above 3,000 and a cylinder-8 attribution using twenty percent of that weighted total. The raw and weighted quantities should be kept separately when documenting the event.

A verification record should include execution

The monitor requires valid calculated segment timing and appropriate combustion conditions. Fuel cutoff, zero-torque detection, and the startup delay influence whether observations are eligible. Time spent with the engine running is therefore not automatically time spent completing this diagnostic.

Two-trip reporting is specified. A useful confirmation shows the relevant test frame completed with acceptable cylinder-8 behavior and trustworthy timing information. The table reports combustion evidence but provides no basis for selecting a coil, injector, or mechanical repair without further diagnosis and correct cylinder identification.

Documentation and applicability

These details describe the monitor in GM’s 2025 25OBDG07A Part2 file. Its application-matrix filename does not match the download index, so model and engine applicability remain unresolved. Use the vehicle’s correct service information before applying these calibration details.

GM diagnostic table, PDF page 152; GM diagnostic table, PDF page 153.