P074B: Third-gear engagement failure needs speed and state history

P074B identifies inability to engage third gear. The cited GM table evaluates repeated attempts, using both synchronizer speed relationships and the controller’s actuation-state progress. A final gear display alone cannot reveal which condition failed.

The speed limits depend on the stage

At the beginning of engagement, the comparison uses the applicable maximum initial differential-speed limit plus 250 rpm. During engagement, another limit belongs to the current actuation state. These settings are referenced in calibration summary attachments.

A transition that does not become eligible before its timer expires supplies another failure path. The third-gear request and actuation must be active, system pressure must exceed 4.5 bar, and error or bypass states are excluded. Fault confirmation requires at least four failed attempts.

Keep the third-gear event distinct

Retain the requested gear, differential-speed trace and failed state. A failure to leave third gear would be a disengagement problem with different logic; it should not be substituted for the event P074B describes.

The summary does not prove a defective synchronizer or identify a pressure-control component. It provides a structured description of unsuccessful engagement, not a complete workshop test. Consult the applicable transmission calibration for exact limits and approved diagnostic actions. Do not interpret the 250-rpm offset as the entire permitted differential speed or repeat shifts solely to provoke another failure.

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 1043; GM diagnostic table, PDF page 1044.