Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
It is possible sometimes for these remotes to lose memory when the battery is replaced requiring reprogramming, however it usually only occurs I'd the old battery is extremely weak... If the old battery was still working out is less likely.
The easy way to tell if this happened is by looking at the red light on the remote, as this is an indicator of if the problem is related to they new battery or the key itself.... If the remote worked with the old battery and not the new battery and the light works, odds are they memory wad lost. If the remote did not work before and the light does work, then it is possible the actual remote is at fault.
If however the light does not work, then we would suspect the new battery may be bad (happens more than you would expect) or there is a contact issue with the battery installation (upside down, metal tabs bent, etc).
So with all that said, does the light on the key remote work?
It looks like we missed each other; if you need further help with this I will be here for several hours yet tonight, just let me know how I can help.