P2864: Why rod 5 position can disagree with an engaged gear

P2864 is more than a fork-position voltage check in this GM transmission. The controller compares rod 5 position with independent evidence that its B-side gear has actually engaged. A sensor can stay electrically valid yet report a mechanical state inconsistent with clutch and output speeds.

How the contradiction is detected

During the documented engagement check, the position reading differs from the learned engaged B position by more than 1,100 micrometres. The decision also requires hydraulic power, an active rod 5 engagement and a force target above 600 N. Speed information must indicate engagement, and relevant electrical faults must not undermine that information.

A separate disengagement comparison asks whether the position signal remains near its earlier state even though speed evidence says the gear has disengaged. The source also describes a static check around learned positions. These are different opportunities to expose a consistency failure; one parked position reading cannot reproduce all of them.

Useful diagnostic evidence

Preserve the commanded shift phase, learned position, actual position and speed-based gear assessment together. If the speed inputs are unreliable, the independent cross-check is weakened. If the position is wrong only during engagement, distinguish that from persistent drift without force.

The table supports investigating contradictory measurements, not automatically replacing the fork or sensor. Hydraulic and internal transmission testing require the matching service procedure; the listed force is a monitor condition, not an instruction to apply external force.

Source and applicability

2025 GM 07A-2 documented monitor only; module and calibration applicability must be matched to the vehicle. The application-matrix filename join remains unresolved; no model or engine fitment is inferred. GM PDF, page 971. GM PDF, page 972.