Found a blown 3A fuse, tech replaced the fuse, waited to test heat mode and found thermostat malfunctioning with a Y (compressor) call, only the emergency heat engaged, bad thermostat was diagnosed. Costumer had a spare thermostat on hand, replaced it with the existing one and was malfunctioning as well. Homeowner invested in a Honeywell focus pro 5000, tested heat and compressor operated, tech checked the refrigerant levels due to the compressor making an abnormal noise, refrigerant levels were low based on pressure chart, pressures of 205/20. Tech provided the option to perform a leak search or add refrigerant, costumer decided to just add refrigerant. System took approximately 3lbs of R410a to acquire proper specs in heat mode. Costumer decided to invest into our Premium service agreement as a monthly option. System is back heating. Refrigerant levels could drop due to potentially having a leak in the system.