Arrives for a furnace showing that it was overheating. I found a filter clogged with white flakes from the standalone humidifiers. Customer had an extra filter so we swapped the filter out and could hear the airflow slow down in the system. We changed out humidifier pad with customer provided humidifier pad. I tested high limit switch and it passed the test. UV ballast is not working properly. The ballast is receiving voltage, but is not sending the voltage to the bulbs. I checked the bulbs and the bulbs are in good condition.