P077BOutput Shaft Speed Sensor Circuit Direction Error
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- Some transmissions use an output shaft speed sensor that also reports direction of rotation: in a GM patent two offset Hall elements send one pulse per tooth whose width encodes forward or reverse. The direction is needed to detect rolling back on a hill (a Hyundai patent raises idle speed then), for hill hold, stop-start and the forward/reverse change. P077B means the sensor reports a direction that does not fit – for example reverse while a forward gear is engaged and the input and intermediate shaft sensors confirm forward motion. GM's transmission modules list P077B as a blocking condition for other checks, but run the actual check under their own codes: direction contradicts the gear for 2.5 s above 500 rpm, after one trip. Speed and direction are then unreliable: hill hold and the park and reverse logic fall back to substitute modes.
- What you may notice
- Check engine light or transmission warning
- Hill hold off
- Limp mode possible
- Speedometer or cruise control unreliable
- Likely causes
- LikelyOutput speed sensor faulty (direction evaluation disturbed)
- PossibleWrong or incorrectly fitted sensor (orientation to the wheel)
- PossibleReluctor wheel damaged, metal debris on the sensor
- RareFaulty reference (input or intermediate shaft sensor)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €150 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.