To fix an invisible fence, make sure that the collar is tight enough to touch the pet's skin, and tighten the battery cover on the collar if necessary. Test the collar with a battery tester, and replace the batteries if the batteries are dead. Check the transmitter for a blinking light, or listen for alarm to determine if there is a break in the underground line. Repair the wires as needed by splicing a new wire into the break.
Allow a finger-width of space between the collar and the dog's skin to ensure the collar isn't uncomfortably tight. If a battery tester isn't available, replace the batteries, and walk over the fence boundary holding the collar to determine whether or not the problem is resolved.
To test the transmitter, disconnect the neutral ends of the wires from the transmitter, and strip the ends of a piece of 12-inch wire. Connect the stripped ends of the piece of wire to each of the fence terminals, and examine the transmitter to see if the beeping or blinking light stops.
If the dog collar works near the loop, and the malfunction indicator stops, examine any exposed line, and identify areas of disturbance, such as holes, near the underground line. Dig the line up in those areas to locate the break. Remove the damaged wire, and splice new wire into the line to repair the fence.