P215B: speed correlation can fail because the trusted source is unavailable

P215B compares vehicle-speed information from the wheels with vehicle speed derived from transmission output-shaft speed in the reviewed GM strategy. It also has an availability branch. The code therefore does not always mean that two available speed readings differed.

Two distinct evidence paths

The correlation branch fails when the absolute difference between the two vehicle-speed estimates exceeds 6.21 mph. These are both expressed as vehicle speed; a raw shaft rpm value cannot be compared directly with the mph threshold without the system's conversion.

The alternative branch detects that the secure vehicle-speed source is unavailable. This distinction changes the next question: investigate disagreement between credible sources when both exist, but investigate source availability when the controller has no acceptable secure value.

Conditions and confirmation

The source requires more than 0.5000 second since initial CAN activity and an engaged transmission state, with the secure source selected from the output-shaft or wheel-speed information for the relevant comparison. Qualification uses 400 counts within 800 at 25 milliseconds per count. Reporting is Type A over one trip.

Verification should retain both speed estimates, their validity and the selected-source status throughout an eligible interval. A dashboard speed reading alone cannot show whether the two sources agree or which one the controller trusted. The monitor does not identify a particular wheel sensor, output-speed sensor or mechanical component for replacement. These thresholds describe only the documented 2025 GM implementation; transmission and calibration applicability require separate confirmation.

Source: GM monitor, PDF page 790.