P0826: manual-shift input voltage represents no legal switch state

P0826 diagnoses an illegal, out-of-range upshift/downshift switch state in the cited TCM. The controller cannot interpret the input as one of the permitted states. This differs from a valid tap-up or tap-down state that remains active too long.

Duration and supply matter

The source requires the illegal state to persist more than sixty seconds, updating every one hundred milliseconds. A brief transitional reading therefore does not automatically satisfy this monitor. The table also lists run/crank supply prerequisites and excludes the specified P1761 fault.

No numeric legal switch-voltage bands are supplied on the page. Consequently, this article cannot provide resistor-ladder specifications, a pin voltage chart or a substitute direct electrical test. Those depend on the actual switch architecture.

Why manual shifting may disappear

The documented emissions-neutral fallback disables tap-up/tap-down or manual-up/manual-down functionality. If those controls stop responding, the symptom can be consistent with the controller rejecting an invalid input rather than evidence that the transmission's mechanical shifts are impossible.

Record the interpreted switch state and whether the illegal indication persists with the controls released. That comparison helps distinguish an out-of-range state from an ordinary held command. Verification should show valid switch states through normal operation and a passing enabled diagnostic after the applicable repair procedure. The code does not by itself prove which switch, connection or controller interface is faulty, and clearing it alone does not validate the input.

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 1128.