If this has the 97 era HVAC display, go to the data screen ( key on / display showing that the HVAC is on and fan requested to run / Press and hold the REST button until you see a alternating " 01 " and a " value " .


Using the fan + and - buttons, scroll until you find the fan control voltage ( or similar ) display. ( I don't have chart handy. )

At the fan controller, measure voltage between the small signal wire and ground, it should match the voltage on the display. If not, find this wire on the HVAC control and measure voltage there. If you have voltage at the HVAC control, the signal wire is broken.