P077C code meaning
Output Shaft Speed Sensor Circuit Low
Powertrain system Generic code P07xx codes Check engine light
What P077C means
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.
P077C at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor1 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P077C
- Check engine light at once (GM)
- Limp mode or fixed gear
- Speedometer or cruise control failed
- Harsh shifts, no converter lock-up
Common causes of P077C
- Most likelySignal or supply wire shorted to ground (chafing at the case)
- PossibleSensor shorted internally
- PossibleOil or moisture in the connector, corrosion
- Less likelyModule input faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Output speed sensorSensor wiringTransmission control module
What each part of P077C means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 7 | Area: transmission. |
| 7C | The exact fault in that area. |
What to do next
- Note P077C and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P077C
What does P077C mean?
P077C means "Output Shaft Speed Sensor Circuit Low". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P077C?
Drive gently and only to a workshop. With P077C the car can go into limp mode (reduced power) to protect itself.
Will P077C fail an emissions test?
Yes, it can. P077C is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P077C to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P077C clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.