Hi

One of the keys to our forester has an intermittent problem locking or unlocking the car - looks to me like the battery needs replacing in the fob. Is is common? It looks to be a simple DIY job. Batteries are cheap on ebay.

Its a UK car so its the chunky key with a single lock/unlock button. Single screw opens the fob and the battery is easy to extract.

 

Depending on the year, it may be suffering from "radio lockout". Search for that, but a quick description is that if you are near a hospital/police station etc, their radio signals can stop the fob working.
It probably is just the battery though. It pops out easy once the screw is undone.