P077AOutput Shaft Speed Sensor Circuit Loss of Direction Signal
- 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. P077A means the sensor still delivers speed pulses but no usable direction information – the pulse width stays outside the direction windows or one Hall element has failed. No manufacturer document available to us uses this code; GM checks direction coherence under P0721. Hill hold, auto hold or stop-start may then be disabled and the forward/reverse change runs in a substitute mode.
- What you may notice
- Transmission warning or check engine light depending on manufacturer
- Hill hold, auto hold or stop-start unavailable
- Rolling back on a hill not caught
- Likely causes
- LikelyOutput speed sensor faulty (one Hall element or direction channel failed)
- PossibleContact resistance in connector or wiring distorts the pulse
- PossibleWrong replacement sensor without direction function
- RareReluctor wheel damaged or air gap too large
- 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.