Some common trailer brake controller wiring problems include an erratic display while braking and the brake controller losing connection with the battery. Other common problems include the display reporting an overload and the controller not turning on.
Further common problems include the display not showing that the controller is connected and the brakes applying too strongly for their settings or engaging too soon after connection. Trailer brakes only working when operated manually is also an issue often attributed to faulty wiring.
Etrailer.com provides information on possible causes of these common problems and a list of procedures used to test for those causes.