Hyundai Sonata harsh downshift issue at 20 mph requiring potential computer reprogramming and warranty transmission replacement on Hyundai Sonata

Some Hyundai Sonata owners report experiencing harsh downshifting issues at around 20 mph, which may stem from a faulty 1-2 shift solenoid or a torque converter clutch that fails to unlock properly. While some drivers have found that resetting the adaptive values provides a temporary fix, the problem often recurs. Additionally, several Sonata owners have noted hard thumps during downshifting, indicating a persistent transmission issue. For those with a vehicle under warranty, it is advisable to consult a certified mechanic or dealership to address the transmission concerns, although Hyundai's warranty coverage may have exceptions. Professional assessment is recommended to accurately diagnose the cause of the harsh downshifting and determine if computer reprogramming or warranty transmission replacement is required. Prompt attention to this issue is crucial for ensuring safe and reliable vehicle operation.