Upon arrival, I identified that the unit was not functioning due to the absence of a filter. I began by installing a new filter, ensuring it was correctly positioned and securely in place. Next, I inspected the flame sensor, which was found to be dirty and obstructed. I meticulously cleaned the flame sensor using appropriate tools and techniques, removing any buildup that could hinder its performance. After these steps, I conducted a thorough check to confirm that all components were functioning properly. The unit is now operational, with the filter in place and the flame sensor clean, ensuring efficient and safe performance.