Powertrain, generic · Transmission

P077B Output Shaft Speed Sensor Circuit Direction Error

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. 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.

Likely causes

Output speed sensor faulty (direction evaluation disturbed)
Wrong or incorrectly fitted sensor (orientation to the wheel)
Reluctor wheel damaged, metal debris on the sensor
Faulty reference (input or intermediate shaft sensor)

Symptoms

  • Check engine light or transmission warning
  • Hill hold off
  • Limp mode possible
  • Speedometer or cruise control unreliable

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

P077AP077CP077DP0721P0722P0723

References: gsi.ext.gm.com, gsi.ext.gm.com, patents.google.com, patents.google.com.