To clear a fault on a LynxR-2 or a Vista 20p you would simply enter in your master code then hit the off button, you would do this two times in a row to clear the fault.
If this does not clear the error, this means that the device is still open (or the cover is).
If the cover is on correctly, then check the loop number on the device in programming. Refer to your sensor specific instructions on which loop to use. Keep in mind that if you are using the built in reed switch on the 5816 or 5815, you need to program loop 2.