Go to dealer or check glove box, it has the code inside.
Dead cell in the battery itself which requires you replace the battery. Can also be any light on the vehicle that is on pulling power from the battery. It can also be a relay that is stuck closed.
alarm company says to change yearly
Clock & radio presets. Possibly the radio security code if the car is equipped with a security system on the radio.
If you need a battery then there is nothing that can replace it.
if the battery gets weak and useless, then you have to replace it...
how to replace battery
how to replace a mitsubishi endeavor battery
most likely it's a dud battery in fob. Easy to replace, just prise open fob. Keep a spare battery somewhere as I have been locked out by a dud battery!
no
how to replace a 1990 Chevy Beretta battery
Lift up rear seat & replace battery
Remove the seat. Remove the battery. Replace the battery. Replace the seat.