P0791 Fault Code in BMW: Causes, Symptoms, and Fixes

This code indicates that the Transmission Control Module (TCM)** has detected a problem with the intermediate shaft speed sensor “A”. This sensor monitors the rotational speed of a shaft inside the transmission, typically used to calculate gear ratio and support shift timing.

Symptoms:

1.️ Check Engine Light (CEL)
2. Hard or delayed shifting
3. Transmission may enter limp mode
4.️ Harsh gear engagement or slipping
5. Gear display malfunction (e.g., wrong gear shown)
6. Poor acceleration

Common Causes:

1. Faulty intermediate shaft speed sensor
2. Damaged or corroded wiring/connectors to the sensor
3. Short to ground or open circuit in sensor wiring
4. Faulty or failing Transmission Control Module (TCM)
5. Internal transmission issue causing abnormal shaft speed

Recommended Fixes:

✅ Step-by-step:
1. Scan with advanced diagnostic tool (BMW ISTA, VCDS, or equivalent)
– Check live speed sensor data (input/output/intermediate)
2. Inspect and test the sensor:
– Use a multimeter to check for signal and power
– Look for resistance out of spec or signal dropout
3. Inspect wiring and connector
– Look for corrosion, bent pins, damage, or loose fit
– Clean or replace as needed
4. Replace sensor if faulty
– Sensor is often mounted inside the transmission or near the valve body
– Some require removing the transmission pan to access
5. Clear codes and test drive

Found in:

1. BMW (6HP, 8HP transmissions)
2. Audi / VW DSG (DQ250, DQ200)
3. Hyundai/Kia DCT
4. Ford Powershift
5. Toyota/Lexus automatic gearboxes