P077B
Output Shaft Speed Sensor Circuit - Direction Error
Transmission (speed sensor)
Generic OBD-II
Medium
What it means
P077B means the output shaft speed sensor is reporting the driveshaft turning in a direction that conflicts with what the transmission expects — for example, signalling reverse rotation while the car moves forward. The control module can't trust the speed reading, so it may shift erratically, show a wrong speedometer, or drop into a failsafe mode.
Medium
Can I keep driving? Usually drivable, but the transmission may shift erratically or enter limp mode and the speedometer can read wrong — get it checked soon.
Common symptoms
What you'll notice
- Erratic or harsh shifting
- Transmission may enter limp/failsafe mode
- Inaccurate or dropping speedometer
- Check-engine light on
Common causes
Most likely first
- Faulty output shaft speed sensorLikely
- Damaged wiring, poor connector, or corrosion in the sensor circuitCommon
- Metal debris or a damaged reluctor/tone ring at the sensorCommon
- Faulty transmission control module (less common)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame and inspect the output speed sensor wiring and connector.
- Check the sensor tip and reluctor ring for metal debris or damage.
- Measure the sensor's output/resistance against spec.
- Replace the output shaft speed sensor if faulty.
- If the signal is still wrong, check for internal damage or evaluate the TCM.
FAQ
Quick answers
Is it safe to drive with P077B?+
Usually yes for short distances, but erratic shifting and a bad speed signal can drop the transmission into limp mode, so have it looked at soon.
Can I just clear P077B?+
It will reset once the sensor reports a conflicting direction again; the sensor, its wiring, or the tone ring needs to be repaired first.
Related codes
Often seen with P077B
Read & clear P077B yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX