Troubleshoot on the schematic/wiring diagram to find the defective thermostat. Order a replacement part. Remove all outer covers to gain access to the thermostat. Only tool needed for this is a #2 phillips screwdriver. Cut the leads on the defective part and remove thermostat and replace thermostat, wire cutters/crimper required, splice the thermostat leads, Reassemble the outer covers.
The switch housing was not exactly the same for easy snap in. I removed the actual switch and installed it into the old housing. Then I found that the male pins were larger than the old ones. I chose to clip off the female connectores, strip the wires, and solder the wires to the pins on the switch. It was easier for me than attempting to change the push on connectors. For me, the actual switch was what I wanted and I fixed it my way. It was easy to do however some folks may want to change the push on connectors. Works great and saved me hundreds of dollars. Thanks.
