Ultimately, you may have to resort to a locksmith to have the key fob re-synced with the car. But before you do that, try this: Mitsubishis of this era often have a key fob with a lock and unlock button. While sitting in the car (or standing next to it) push both lock and unlock buttons down together. Hold them down until you see the fob’s LED flash three times. Then, push the unlock button for one flash of the LED. The LED should then flash another two times and you’re all set. The lock and unlock buttons should now work. It may not work with all cars, but it’s definitely worth a try. If it doesn’t, you probably have a Mitsubishi model where syncing has to take place via the OBD (On-Board Diagnostics) plug which requires a bit more know-how and the correct plug tools. That’s where your friendly locksmith or auto electrician comes in.