P0721: directional output-speed plausibility needs more than RPM

P0721 evaluates directional output-speed information in the cited TCM implementation. It compares the sensor's direction-related on-period information with raw output speed and the distinction between transitional and nontransitional motion.

A valid speed magnitude can hide a direction problem

The table explicitly applies to a directional sensor configuration. One documented failure path is raw speed above 225 rpm while the signal identifies a transitional period. Thus, an apparently believable RPM value does not by itself establish coherent direction information.

The other path concerns a nontransitional state whose raw direction is neither forward nor reverse. However, the strategy prose contains contradictory wording about the expected direction in that state, and several period-bound expressions are awkwardly rendered. Exact pulse-period limits are therefore left to the vehicle-specific procedure.

Calibrations also differ in reporting

Both supplied examples use more than 3.5 seconds of failure in a sample period exceeding five seconds. The 03A source classifies the diagnostic Type B/two trips, while 04B lists Type A/one trip. A generic code cannot determine the reporting behavior.

Capture direction status, transition status and raw speed together and confirm the installed sensor configuration. Verification must establish coherent direction and magnitude under the applicable enabled test. This is not a universal output-sensor low-voltage test or proof of a failed gearset. The source wording conflict remains flagged rather than converted into an invented waveform specification.

Source and applicability

Documented monitors in gm-2025-03a, gm-2025-04b, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 1102. GM 2025-04b, page 2055.