Upon arrival unit was on but not running. Customer stated the fan does not always turn on when here is a call for heat and they cycle power for the unit and it seems to fix it. Turned thermostat up and checked for power at the heating relay. Relay pulled in and had power but the fan never turned on. We found the relay was not always allowing power through to turn the fan on. Replaced the relay and checked for correct operation. Unit is working properly.