Upon arrival performed a one system maintenance checking heating. Checked inside air handlers heat strips, electrical and mechanical components, blower amps and capacitor. Checked filter. Checked outside units pressures amp draws and capacitors. All looked good. However when testing system in heating mode tried to force defrost and system would not go into defrost. Bypassed board to try and trick system into defrost and system would not go into defrost mode. Board is not turning off fan motor and performing defrost mode. Board is under warranty and in stock. Emailed estimate over to warranty labor out defrost board.