2004 LS trasmission problem

Hello,

it's the third time it's arrive. When i put the shifter in L, it's impossible to downshift or shift with the switch on the shifter. I park my car and put on P after D and now it's again possible to downshift or shift with the swith. It's very heratic. I have told with my dealer and this answer is very simple... if the check engine has'n lit and can't do nothing.

thanks for you help

No clue.