Changed battery on my key fob and it won't work

When was the key fob battery changed last?

Take it to a radio installer, car alarm specialist to check the operation of the fob, reprogramming and alarm system check.

Be sure to use the correct battery, properly in place, clean the key contacts , See owner's manual for additional information.
There are specialized locksmiths for car keys

First things first...replace the cheaper part first, the battery. It that works, great! If not, then you may need a new key fob then have it programed to your system.

you may have the battery in backwards

There is a special procedure involved in keying your fob to your car once the battery has been changed. You must lock and open the door with your key. Turn the ignition to "on"(not start) then push the fob's open and close button, then shut off ignition. This procedure is done everything the key battery is changed. Hope this can help you!
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Motorcycle Diagnostic Scanner

It is not clear whether the problem is with the fob or the receiver in the car. You could try replacing the fob (they are available online from various sites) but the receiver will need to be programmed to use it, and that usually requires a trip to a dealership (unless you have access to the OEM service manual). Your best bet is to take it to a dealership and have them diagnose and repair the problem.

Inside the key fob are contacts that get dirty, both the electric side and the push button side, carefully clean them both.

You have to take it and get it reprogram. The pcm is not able to remember it.

HAVE YOU TRIED CHANGING BATTERY IN FOB

can you manually unlock the lock and open the door?? Is it more than one door or just one specific door? If its just one door, yourcactuator inside the door may be bad or you have a wire issue. If its all the doors, more than likely you blew a fuse or a relay(id say fuse)