My remote control was becoming slower and slower to respond, and it got to the point where I had to press it multiple times for it to open/lock the car.

 

I changed the battery, and at the time I remember having some trouble getting it to work. Initially it refused to work and then after pressing the buttons a few times it started to work. This was in early August.

Then last week I went out; when I unlocked the car it worked perfectly but when I came to lock the car the remote control was completely dead. No gradual degradation like I had last time, and the battery was changed only a couple of months ago. I was able to lock the car with the key, but when I unlocked the car it set the alarm off!

 

I suspect the battery is bad, but it's strange that it failed so suddenly, and so soon, as opposed to the previous battery which wore out gradually over a much longer period of time.

 

Any thoughts? (And is there any other way to turn off the alarm if the remote control isn't working?)

I would try a replacement (quality one) in the fob and see if that improves it

 

I measured the battery voltage with a multimeter. It was around 2.3V when it should have been 3.3! I guess one of the cells inside the battery must have suddenly failed to cause such a sudden voltage drop.

 

I replaced the battery with another of the same batch and so far, so good.