Properly Performed spring/fall maintenance in whole home system. Checked the filter checked blower wheels for built up dirt/dust, took delta Ts, cleared drain lines, tested safety switches, tested all electrical components, tightened all electrical connections, took running volts/amps, monitored systems operations. System is not cooling due to bad defrost board. After replacing the defrost board, system was still acting. The thermostat was only giving 24v to the Heatpump Orange wire on cool mode, causing system not to cooling due. After diagnostic found that the neste thermostat was bad. Bad Nest Thermostat Dirty air filter. Dirty condenser coil due to dryer vent blowing straight on Heatpump. We replaced the thermostat and the defrost board. Tested system for normal operation, system is heating and cooling pro at this time.