Came out to call. Customer stated that the heater hasn’t worked at all. They checked the system and saw an led light flashing, but nothing happening. System was installed in the fall of 2019. I checked system over and found the furnace led code stating the furnace had reverse polarity. I checked power wire and reversed the power legs on the furnace. Cycled system on and the furnace started operating properly. Filter was dirty and I recommended it be replaced. Customer had me change it. I also recommended maintenance plan to customer seeing they are first time home buyers and it would be one less thing to worry about.