P0735 Fault Code in BMW: Causes, Symptoms, and Fixes

P0735 indicates that the Transmission Control Module (TCM) has detected an incorrect gear ratio when the transmission is in 5th gear. The actual gear ratio (calculated from input and output speed sensors) doesn’t match the expected ratio for 5th gear, signaling a slipping or shifting issue.

Common Causes:

1. Low, dirty, or degraded transmission fluid
2. Faulty shift solenoid (affecting 5th gear)
3. Worn or damaged clutch pack (specifically for 5th gear)
4. Faulty input or output speed sensors
5. Internal transmission failure
6. TCM software issue or malfunction
7. Damaged wiring or corroded connectors

Symptoms:

1. Transmission slipping or flaring in 5th gear
2. Harsh or delayed upshifts/downshifts
3. Vehicle may skip 5th gear or go into limp mode
4. Poor highway performance or fuel efficiency
5. Check Engine Light (CEL) illuminated
6. Possible vibration or abnormal engine revving at high speed

Diagnosis and Fixes:

1. Check transmission fluid level and quality
2. Replace fluid and filter if dirty or old
3. Scan live data for input/output speed sensor readings
4. Test and replace speed sensors if values are erratic
5. Check and test shift solenoids
6. Inspect wiring to transmission, sensors, and TCM for damage or corrosion
7. Update or reflash TCM software
8. Transmission rebuild or replacement if internal damage is found (e.g., clutches, bands)

Common in These Vehicles:

This fault often appears in vehicles with 5-speed or 6-speed automatic transmissions, including Honda, Jeep, Ford, Hyundai, BMW, Volkswagen, and Audi models.