P0734 Fault Code in BMW: Causes, Symptoms, and Fixes

P0734 indicates that the Transmission Control Module (TCM) has detected an incorrect gear ratio when the transmission attempts to operate in 4th gear. This means that the transmission input speed and output speed don’t match the expected values for 4th gear.

Common Causes:

1. Low or contaminated transmission fluid
2. Faulty shift solenoid (related to 4th gear)
3. Worn or damaged 4th gear clutch pack
4. Faulty input or output speed sensors
5. Internal transmission damage (bands/gears)
6. TCM malfunction or software error
7. Wiring issues (shorts, opens, or corroded connectors)

Symptoms:

1. Slipping in 4th gear
2. Harsh or delayed shift to 4th
3. Transmission may skip 4th gear entirely
4. Poor fuel efficiency at highway speeds
5. Check Engine Light (CEL) illuminated
6. Vehicle may enter limp mode

Fixes / Diagnosis Steps:

1. Check transmission fluid level and condition (burnt or dark = bad)
2. Replace or clean transmission fluid and filter
3. Scan for live data to verify input/output speed sensor readings
4. Test and replace input/output speed sensors if faulty
5. Test shift solenoids with multimeter or scan tool
6. Inspect wiring and connectors to TCM and sensors
7. Reprogram or update TCM software
8. If mechanical damage is found: rebuild or replace transmission

Affected Vehicles:

Seen commonly in Honda, Hyundai, Jeep, Ford, BMW, Audi, and Volkswagen models with automatic transmissions. Particularly on 4-speed or 6-speed automatic gearboxes.