Came out to call. Customer stated that the tenants to her that the system wasn’t heating properly. They moved into the rental property in November. I checked system over and found the furnace running in lockout, and the fan was running continuously. I checked filter and found it severely clogged, and it hadn’t been changed since April. New tenants had no idea where the filter was located or what size. I showed tenant where it was located and how to change it. I also recommended it be changed every 2-3 months. I found the flame sensor also needing to be cleaned. Cleaned the flame sensor and reinstalled it. Cycled system on to test and verify that the furnace is operational. Furnace is now heating properly.