The gas valve in a furnace controls the incoming gas supply that fuels the heating process. The valve helps ensure that the furnace only receives the gas it needs, which prevents energy losses and potentially hazardous gas leaks. If the valve stops operating correctly, your furnace could quickly stop heating at all.
Did you narrow the problem down to the gas valve through troubleshooting? Removing and replacing the part isn't difficult though you should always feel comfortable calling in a heating contractor to perform the task instead.
If your air-conditioning system could select a most valuable player, the compressor might win the title. The motor-driven compressor sits at the start of the cooling process and compresses gas refrigerant out into the system to provide the fuel for your indoor climate change. If the compressor stops working correctly, your entire HVAC system will suffer.
Compressors can suddenly start experiencing problems starting up or staying running for extended periods of time.