Lexus introduced the ES300 in the 1992 model year. The 2003 model year marked the final year for the ES300, as the maker changed from the 3.0-liter engine to the 3.3-liter engine and renamed the luxury sedan the ES330. The 2003 ES300 came standard with a keyless entry feature, but unlike older keyless entry systems, Lexus integrated the key fob with the ignition key assembly. In order to change the fob's battery in the 2003 Lexus ES300 you need to take the top of the key assembly apart.
Remove the screw on the key side of the key fob, using a small Phillips screwdriver. Slide the cover away from the key to remove it, exposing the module inside the top of the key assembly.
Pull the key fob module from inside the key fob.
Remove the two screws on the key fob module, using a small Phillips screwdriver, and pull the cover from the module, exposing the flat, circular battery inside the module.
Pry the battery out of the key fob module, using a small flat-head screwdriver. Set a new 3-volt battery in the key fob, positive side up, and press it down to lock it in place.
Reinstall the key fob module lid and tighten the two screws with a Phillips screwdriver. Set the key fob module back inside the key fob.
Slide the key fob cover back into place and tighten the screw with a small Phillips screwdriver.
