Arrived to system off. Put system in heat mode and went to indoor unit found blown fuse. Replaced fuse and turned fan on system ran, turned on emergency heat system ran, turned on heat and fuse blew unwired 24v to outdoor unit and tested system in heat indoor unit ran. Found rub out on low pressure switch. Wrapped wires in electrical tape to prevent wires from shorting out. Tested system in heat, and found that we were not getting 24v from contact on the defrost board. Spoke with homeowner need to get a senior tech out to confirm bad defrost board. System is operating in emergency heat now.