I have a 3yo MasterTemp300 with the heat cycling on/off about every minute. The 'SERVICE SYSTEM' LED is not on, and no LEDs on the backside of the board are illuminated. My first guess was that the thermal regulator was not functional, but it has been replaced without success. The troubleshooting flowchart in the manual suggests that the Control Board needs replaced, but I'm hesitant to move forward without more confirmation. Any suggestions about how to proceed or other troubleshooting measures to try? Could this be an issue with the manifold bypass valve? Any help is appreciated!