P0816: a recognized downshift request does not release

P0816 identifies a downshift input stuck active in the cited TCM strategy. The controller recognizes the tap-down state, but its duration fails the applicable timing test. The published fallback disables tap/manual up and down operation.

Recognized state versus out-of-range state

This monitor is distinct from P0826's illegal switch voltage. The displayed enable list excludes P0826 active, pending and failed-this-key-cycle results. An invalid electrical state should therefore not be treated as if it were simply a valid downshift button held too long.

For the first shown branch, the downshift state exceeds one second with a qualifying forward range and sufficient time since a selector-range change. The source begins another branch with a 120-second threshold. Its remaining conditions continue beyond the displayed page, so one universal timeout is not justified.

Interpreting the symptom

Loss of manual downshift availability can match the documented fallback, but it does not establish mechanical inability to change gear. Capture the input state during button release, selected range and the diagnostic branch where available.

Verification should demonstrate that the request returns to its appropriate inactive state and that the relevant enabled monitor passes. A successful automatic shift alone does not validate this manual-request input. Use the exact switch and transmission service procedure before assigning a replacement. The source supplies no switch location, resistance ladder values or repair-likelihood ranking, and classifies the diagnostic as emissions-neutral Type C.

Source and applicability

Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 1126.