P0791
Intermediate Shaft Speed Sensor A Circuit
Transmission — speed sensing
Generic OBD-II
Medium
What it means
P0791 means the control module has detected a circuit problem with the intermediate shaft speed sensor. The transmission uses this sensor to compare shaft speeds and confirm it is in the right gear, so a bad signal can cause erratic or delayed shifts and may force a protective limp mode. It's a sensor fault — it affects shifting, not engine starting.
Medium
Can I keep driving? Often driveable but shifting may be erratic or the transmission may limp — drive gently to a shop; it won't stop the engine from starting.
Common symptoms
What you'll notice
- Erratic, delayed, or harsh shifting
- Transmission slipping or dropping into limp mode
- Wrong-gear engagement
- Check-engine light on
Common causes
Most likely first
- Failed intermediate shaft speed sensorLikely
- Damaged wiring or corroded connector in the sensor circuitCommon
- Debris or damage on the sensor's tone/reluctor ringCommon
- Loose or corroded electrical connectionsLess often
- Failed transmission control module (rare)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and compare the intermediate shaft speed reading against input/output speeds in live data.
- Inspect the sensor wiring and connector for damage or corrosion.
- Check the sensor tip and tone ring for metal debris or damage.
- Measure the sensor and its circuit; replace the sensor if it reads open, shorted, or erratic.
- Clear the code and road-test to confirm normal shifting returns.
FAQ
Quick answers
Is it safe to drive with P0791?+
You can usually limp to a shop, but without a valid shaft-speed signal the transmission may shift erratically or lock in one gear. Avoid long trips.
Can I just clear P0791?+
It returns once the sensor or wiring fault reappears. Diagnose the circuit rather than only clearing the code.
Related codes
Often seen with P0791
Read & clear P0791 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