Upon arrival, I identified that the heating unit was not operational due to a tripped main limit fault. The filter was found to be excessively dirty, which I promptly replaced. After ensuring the system was functioning correctly, I observed that the unit successfully reached the desired temperature. During a thorough inspection, I discovered mold in the humidifier, which I informed the customer about. To ensure safety, I took a carbon monoxide reading of the system, confirming it was within safe levels. The unit is now fully operational and providing efficient heating.