P07D8: Eighth-gear engagement failure depends on speed and actuation progress

P07D8 is the unable-to-engage-eighth-gear code. In the cited GM strategy, the controller follows the engagement attempt and records why progress can fail. It does not diagnose the transmission from a single gear request alone.

The differential-speed limit is calibrated

The initial check compares synchronizer differential speed with a gear-specific limit plus 250 rpm. Another check uses the maximum allowed differential speed for the current engagement state. A failure to meet the next state transition before its calibrated timeout supplies an additional path.

Eighth-gear engagement must be requested and active, system pressure must exceed 4.5 bar, and the application must be outside error and bypass states. Confirmation requires at least four failed engagement attempts.

Preserve the sequence that failed

Retain the eighth-gear request, synchronizer speed difference and actuation-state history. An excessive speed difference and an expired state timer may lead to the same code but point to different evidence for the next service check.

The summary does not identify a failed synchronizer, pressure-control part or speed sensor. Exact state limits are referenced in calibration attachments and are not reproduced as a guessed universal threshold. The 250-rpm addition is only one part of the initial comparison. Follow the appropriate transmission procedure instead of forcing repeated shifts, and distinguish failure to enter eighth from a separate failure to disengage a gear already selected.

Source and applicability

This explanation covers the cited GM monitor, not every vehicle carrying this code. The application-matrix filename does not resolve cleanly to this file, so no model or engine fitment is assigned. GM diagnostic table, PDF page 1045.