Powertrain, generic · Transmission

P077A Output Shaft Speed Sensor Circuit Loss of Direction Signal

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Output speed sensor faulty (one Hall element or direction channel failed)
Contact resistance in connector or wiring distorts the pulse
Wrong replacement sensor without direction function
Reluctor wheel damaged or air gap too large

Symptoms

  • Transmission warning or check engine light depending on manufacturer
  • Hill hold, auto hold or stop-start unavailable
  • Rolling back on a hill not caught

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–500
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

output speed sensorsensor wiringtransmission control module

Related codes

P077BP077CP077DP0721P0722P0720

References: patents.google.com, gsi.ext.gm.com, patents.google.com, law.cornell.edu.