When I arrived, the unit didn’t turn on in heating. I turned thermostat to heating and went to attic to diagnose issue. I saw on error codes the flame sensor needs service or filter dirty. I took out flame sensor and deep cleaned. The flame sensor was white and needed to see flat surface to ignite burners. After cleaning , wiring back up and screwed into existing spot. Furnace took off in heating mode. I also went to check filter and it was very dirty. I replaced air filter with new one. I recommend a hot surface igniter replacement in due time. Igniter is starting to get white and cracked. Only problem is gas valve is directly behind it so that will all need to be dissembled to do work.