P077A code meaning
Output Shaft Speed Sensor Circuit Loss of Direction Signal
Powertrain system Generic code P07xx codes Check engine light
What P077A means
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.
P077A 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 P077A
- Transmission warning or check engine light depending on manufacturer
- Hill hold, auto hold or stop-start unavailable
- Rolling back on a hill not caught
Common causes of P077A
- Most 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
- Less likelyReluctor wheel damaged or air gap too large
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 P077A 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. |
| 7A | The exact fault in that area. |
What to do next
- Note P077A 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 P077A
What does P077A mean?
P077A means "Output Shaft Speed Sensor Circuit Loss of Direction Signal". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P077A?
Drive gently and only to a workshop. With P077A the car can go into limp mode (reduced power) to protect itself.
Will P077A fail an emissions test?
Yes, it can. P077A 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 P077A to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P077A clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.