When the keyless entry remote for your Subaru Forester begins to weaken or fails completely, you should replace the battery as soon as possible. The remote uses a CR2025 battery available from most major retailers and electronics stores. When you replace the battery in your Forester remote, you need to synchronise the remote to the system again for it to work properly.
Insert a flat-bladed screwdriver or butter knife into the indent slot at the wider end of the keyless entry remote. Twist sideways to separate the two sides of the remote and expose the battery.
Lift the old battery out of the remove. Insert the replacement battery with the negative side facing up.
Push the two sides of the remote together until they click, locking together.
Press the "Lock" or "Unlock" button on the keyless entry system six times in rapid succession to synchronise the remote with the keyless entry system.