P077B code meaning
Output Shaft Speed Sensor Circuit Direction Error
Powertrain system Generic code P07xx codes Check engine light
What P077B 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. 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.
P077B 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 P077B
- Check engine light or transmission warning
- Hill hold off
- Limp mode possible
- Speedometer or cruise control unreliable
Common causes of P077B
- Most likelyOutput speed sensor faulty (direction evaluation disturbed)
- PossibleWrong or incorrectly fitted sensor (orientation to the wheel)
- PossibleReluctor wheel damaged, metal debris on the sensor
- Less likelyFaulty reference (input or intermediate shaft sensor)
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 P077B 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. |
| 7B | The exact fault in that area. |
What to do next
- Note P077B 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 P077B
What does P077B mean?
P077B means "Output Shaft Speed Sensor Circuit Direction Error". 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 P077B?
Drive gently and only to a workshop. With P077B the car can go into limp mode (reduced power) to protect itself.
Will P077B fail an emissions test?
Yes, it can. P077B 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 P077B to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P077B clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.