If you have a L light that is stuck on with no car present, you can attempt to do a reset loop to clear the detection. In a 3000 series timer, you will need to do this from the Remote Control function.
Press MAIN MENU
Press the DOWN ARROW until you are on RESET LOOP and press ENTER SELECT
Enter your access code, the default code is 000000.
DOWN ARROW to the event that is locked on and press ENTER SELECT
Before contacting a technician, you may want try a Vehicle detector calibration - LMA1400-MS
If the issue still persists, you may need a technician visit to check your loop and loop cable.