Diagnostic article Generic OBD-II
P074C: fourth-gear engagement exceeds speed or state limits
P074C identifies repeated failure to engage fourth gear in the documented transmission.
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Documented application
2025 GM 25OBDG07A Part2 diagnostic monitor documentation; the application-matrix filename mismatch is unresolved, so no model or engine applicability is inferred.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P074C identifies repeated failure to engage fourth gear in the documented transmission. The monitor checks whether the synchronizer’s speed relationship and the engagement state sequence permit the intended gear to be established.
Distinguish the stage that failed
The initial differential-speed test compares speed with a calibrated maximum plus 250 rpm. Later in the sequence, another state-specific differential-speed limit applies. A separate branch detects an unmet transition condition after the state’s calibrated timeout.
Those three branches should not be summarized as “250 rpm too much slip.” The source delegates the base limits and state timeouts to attachment tables. The 250-rpm value is only an offset in the initial comparison.
Confirmation depends on repeated attempts
Fourth-gear engagement must be requested and active, system pressure must exceed 4.5 bar, and the application must be outside error and bypass states. At least four failed attempts confirm the code.
Preserve the failure stage and associated speed data before selecting a repair. A timed-out state and excessive initial differential speed can lead to the same code while requiring different investigation. Verification should demonstrate successful fourth-gear engagement through the proper service sequence. The monitor does not prove a particular synchronizer, actuator, or hydraulic component is defective, and its retry logic is not an instruction to force shifts.
Documentation and applicability
These details describe the monitor in GM’s 2025 25OBDG07A Part2 file. Its application-matrix filename does not match the download index, so model and engine applicability remain unresolved. Use the vehicle’s correct service information before applying these calibration details.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Unable To Engage Gear 4
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT