Powertrain, generic · Transmission

P077C Output Shaft Speed Sensor Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

GM's transmission modules from 2019 to 2025 (9- and 10-speeds) also read the active output speed sensor's line voltage as an analog value. P077C means this voltage is below 0.25 V – a short to ground of under about 0.5 Ω between signal and module ground – for more than 0.05 s with the system above 10 V; GM rates it after one trip. GM then blocks the no-signal and ratio checks so that P0722 is not set instead of the actual short. Without output speed the transmission enters limp mode and road speed may be missing. On GM's 10-speed input and output sensors share a supply.

Likely causes

Signal or supply wire shorted to ground (chafing at the case)
Sensor shorted internally
Oil or moisture in the connector, corrosion
Module input faulty

Symptoms

  • Check engine light at once (GM)
  • Limp mode or fixed gear
  • Speedometer or cruise control failed
  • Harsh shifts, no converter lock-up

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€120–450
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

P077DP0722P0720P0721P0717P0502

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