To program a key fob, switch on the ignition taking care not to start the engine, set the car into a program mode, press the button on the fob that needs to be programmed, and then turn off the ignition. Test the fob to ensure that it functions properly.
Instructions for reprogramming a key fob vary with the make and model of the car. Prior to programming the fob, carefully refer to the owner's manual.
Put the key into the ignition, and without starting the engine, switch on the ignition in order to set the car into a program mode. Repeat this process if necessary. A car that is in program mode should show indicators such as a flash of lights.
Once the car is in program mode, enter the button on the fob that needs to be programmed in order to synchronize the fob with the car. Turn off the ignition, remove the key and get out of the car. Once out of the car, press the button on the fob to test if it remotely locks and unlocks the car. If the fob fails to function properly, repeat the process as necessary.