Recently our car remote (keychain fob) stopped unlocking the car. It was pretty bad because the locking button still worked, but if you lock the car with the remote & unlock it with the key it sets off the car alarm! (There is also no keyhole on the passenger side.) We were not eager to shell out big bucks for a new one, so I searched online and found an easy fix! A lot of this is going to vary based on your car model, but I'll show you what worked for our HHR remote.
First we're going to crack open the casing. My husband had a difficult time prying it open with knives & screwdrivers and left the edges a bit scratched up. I Googled it & found out there is a little slit at the bottom that you can easily fit a small screwdriver in. Twist & it pops right open!
Splay out the innards. At this point it is a good idea to try the simplest solutions, just in case. We changed the battery, then I cleaned everything out with some rubbing alcohol and Q-tips and tested again with no luck.
Since only one button was effected, the problem was mostly likely that the conductive coating on the back of the button had worn off. This tutorial had a lot of great pictures & tips. He sliced the back off the problem button, sliced the back off a new button (from a cheap calculator or remote) and glued the new backing in place. Later in the comments some people said that you could also use aluminum foil! (ETA - This did not end up working. See note below.)
You can use a mini hole punch to get the perfect sized circle. It won't punch the foil by itself though, so take a small piece of foil and tuck it inside a folded sheet of paper. Separate out your foil circle.
The post suggested using a tiny dab of super glue to attach the foil dot, but it did not hold at all for me. I brainstormed and then grabbed some clear nail polish topcoat! I used a toothpick to apply a small dot of polish, pressed the foil dot on well, and gave it a while to dry.
The aluminum foil fix worked like a charm!! There are also all sorts of replacement buttons, casings, and circuit boards on Amazon if another part of your fob is causing problems.
(ETA - The nail polish did not hold after all, and neither did 2 part epoxy. I ended up slicing off a button from another remote like the tutorial & secured that with some super glue. It held great!)
While I had my polish out, I figured I might as well go all high-school on the remote and personalize it a bit. I filled in the recessed area with a couple coats of Orly's "Halley's Comet" and topped it with the lovely Femme Fatale "Nightsong Grotto" and some topcoat to smooth things out. So sparkly!