If you need to replace the key Battery for the Lexus GS 300, it will depend on the type of key you have. There are two models that have been used since the inception of the GS300. One key has a singe screw in the back, and the second type of key doesn't have any screws in the back. The two types come apart differently to expose the battery inside of the plastic casing.
Remove the screw from the back of the key fob with a precision Phillips-head screwdriver. Place the screw in a location where it will not get lost. The screw is very small.
Press the tip of a small flat-head screwdriver between the metal key and the plastic casing and pry up against the plastic while you push back on the plastic. Do this on the same side that the screw was removed from. The back cover of the key fob will pop off.
Remove the two screws holding the electronic module into the key fob. Again, place the screws where they will not be lost.
Pull the old battery out of the electronic module and replace the battery with Lexus battery CR1616.
Replace the two screws that hold the electronics module into place. Snap the rear cover back into place and secure the single screw that hold the cover onto the key.
Pull the metal key out of the key fob (plastic electronics housing). The metal key is not held in with a screw or glue. It is press-fit into the housing.
Place a flat-head screwdriver into the slot and twist it sideways to separate the two pieces of the key fob.
Pull the old battery out of the plastic housing and replace it with a new Lexus CR1632 battery. The positive side faces upward.
Press the two sides together until they snap in place.
Press the metal key back into the slot until you feel it seat into place.
