A P0734 code in your 2021 Acura MDX indicates that the transmission control module has detected an incorrect ratio in fourth gear. The relationship between input and output shaft speeds doesn't match the expected fourth gear ratio. Your MDX uses a 10-speed automatic transmission that requires precise gear ratio monitoring.
Fourth Gear in Modern Transmissions
With 10 speeds available, fourth gear serves as a mid-range gear for moderate acceleration and some highway cruising in hilly terrain. It handles less extreme torque than lower gears but still requires solid clutch engagement for smooth operation.
Common Causes
The fourth gear clutch pack or related apply device may be worn or damaged. Shift solenoid malfunction affecting fourth gear engagement. Hydraulic issues reducing apply pressure to fourth gear components. Transmission fluid condition affecting clutch engagement quality. Speed sensor errors that manifest specifically in fourth gear calculations.
Symptoms Specific to Fourth Gear
Noticeable slip or flare when the transmission shifts into or operates in fourth gear. Unexpected RPM increases during fourth gear acceleration. The transmission may avoid fourth gear through adaptive learning. Harsh 3-4 or 4-5 shifts indicate engagement problems. Check engine light illumination with P0734.
Acura Diagnostic Approach
Using Honda HDS or a capable scan tool, monitor gear ratio data during a comprehensive road test. Pay attention to fourth gear engagement quality and ratio consistency. Check transmission fluid level and condition. Verify solenoid function for fourth gear apply circuits. Test line pressure during fourth gear operation.