The 2005 Volvo S40 comes with a combo key. This key has a remote keyless entry system integrated with it. If you notice the keyless entry signal getting weak, it's time to replace the battery. When the signal is weak, it may take multiple pushes of the remote buttons to lock or unlock the car's doors. The Volvo's remote uses a basic 3-volt battery you can purchase at almost any electronics store.
Inspect the rear edge of the Volvo S40 combo key. Slide the key blade release latch to the right to unlock the key blade.
Grasp the key blade and slide it out of the combo key to remove it.
Position the combo key flat on a table or other surface. The remote buttons should face downward. On the back cover of the combo key, you'll see a single screw. Remove it with the small screwdriver.
Lift the back cover and set it to the side. Now you'll see the battery. Pry the battery out gently with the screwdriver.
Lay the new battery inside with the "+" symbol pointing down.
Attach the back cover, and then tighten its screw.
Insert the key blade back into its slot. Push it firmly until you hear it latch into place.
