Disconnect loop from detector. Use a multimeter to measure the resistance from one loop wire to ground (stick long nail in dirt if operator does not have ground). Be sure that other loop wire is floating (not connected). If there is more than 8 Meg Ohm leakage to ground loop is faulted. Also check loop continuity (two meter leads to two loop leads) should only read a few ohms at most. Most loop faults occur in wet conditions. If loop passes test replace detectors. You can swap loop detectors if you have two of them to see if the problem moves to other loop.