This is my first real problem with my 2009 Sonata. I really have not had any issues since the time I bought it.
After replacing my failing battery with a new battery from a dealer, who by the way assured me in full compatibility of the battery, one of the terminals on the new battery was disconnecting. The symptom was that after each engine start all of my electronics would reset. Once I opened the hood and jiggled battery terminal the problem became apparent. The terminal design on Sonata is pretty bad, preventing tightening of the terminal clamp around the post. After some research I found battery post shims at Autozone for $0.99 a pair. If I knew about this simple fix, I would have saved a lot of money by purchasing a battery from Autozone instead of Hyundai Dealer. The battery is at least $40 less and dealer does not take core back like auto shops do. After adding shim and fixing the terminal problem, I run in to another problem.
When transmission was cold it would downshift very hard from 4th to 3rd gear. The feeling was like someone rear-ended my car. Initially I thought the issue was with anti-lock breaks but after a visit to a dealer I realized that it was my transmission. Dealer did not fix the issue the first time so the second visit was needed. At the end all they had to do is to erase adaptation tables in the transmission control computer. I suspect that the bad battery connection caused transmission computer to reset frequently and to corrupt these adaptation tables.
Between visits to the dealer I could characterize the issue pretty well and find some work around. Various posts online report the same problem but sometimes they report that downshifting from 3rd to 2nd gear is hard. For newer models of Sonata transmission fluid temperature sensor seems to be the problem. However for 2009 Sonata transmission computer is the problem, and in particular corrupt adaptation tables.
To work around this issue one should switch to manual shifting for the first 10 or so minutes of driving, while transmission is cold. It is sufficient to downshift from 4th to 3rd gear manually when slowing down. The rest of the shifting had no issues. This way you can prevent any damage to your transmission until you have a chance to take it to the repair shop.
After replacing my failing battery with a new battery from a dealer, who by the way assured me in full compatibility of the battery, one of the terminals on the new battery was disconnecting. The symptom was that after each engine start all of my electronics would reset. Once I opened the hood and jiggled battery terminal the problem became apparent. The terminal design on Sonata is pretty bad, preventing tightening of the terminal clamp around the post. After some research I found battery post shims at Autozone for $0.99 a pair. If I knew about this simple fix, I would have saved a lot of money by purchasing a battery from Autozone instead of Hyundai Dealer. The battery is at least $40 less and dealer does not take core back like auto shops do. After adding shim and fixing the terminal problem, I run in to another problem.
When transmission was cold it would downshift very hard from 4th to 3rd gear. The feeling was like someone rear-ended my car. Initially I thought the issue was with anti-lock breaks but after a visit to a dealer I realized that it was my transmission. Dealer did not fix the issue the first time so the second visit was needed. At the end all they had to do is to erase adaptation tables in the transmission control computer. I suspect that the bad battery connection caused transmission computer to reset frequently and to corrupt these adaptation tables.
Between visits to the dealer I could characterize the issue pretty well and find some work around. Various posts online report the same problem but sometimes they report that downshifting from 3rd to 2nd gear is hard. For newer models of Sonata transmission fluid temperature sensor seems to be the problem. However for 2009 Sonata transmission computer is the problem, and in particular corrupt adaptation tables.
To work around this issue one should switch to manual shifting for the first 10 or so minutes of driving, while transmission is cold. It is sufficient to downshift from 4th to 3rd gear manually when slowing down. The rest of the shifting had no issues. This way you can prevent any damage to your transmission until you have a chance to take it to the repair shop.
