P077A — Output Shaft Speed Sensor Circuit - Loss of Direction Signal
The output shaft speed sensor uses a quadrature output (two channels phase-shifted by 90°) so the TCM can determine direction of rotation. The direction channel is missing — TCM cannot tell whether the vehicle is rolling forward or backward, blocking neutral-idle and rollback-protection strategies.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Output shaft speed sensor
- Sensor wiring
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Open direction-channel wire
- Failed quadrature sensor element
Possible symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Diagnostic notes
- The malfunction indicator lamp may illuminate
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.