The receiver went bad, it must have gotten cooked in the hot sun. I went to a local junkyard to find one but couldn't find the exact one i needed. They didn't have and 6th gen montes.

I went to Ebay and found one for around 20 bucks. I bought it and swapped it and programmed it with out and problems. it's been working fine for a few months.

 

If i were at your place, i would probably replace the key fob by contacting an auto locksmith.

 

What Griffindor suggests is very logical but before you go through all that in this heat you can also check that there are no breaks in the battery holder of the key fobs.Mine had this same issue & again recently. I couldn’t believe it but both fobs had the solder connection go bad. A little solder & good as new,if not buy a new fob for 10.00 on Ebay, Program and hopefully all works. Good luck stay cool

 

I got my fob replaced by a mobile locksmith company and they worked great at a decent cost.