Arrived on site to perform furnace diagnostic on 1986 furnace. Upon arrival Jose stated he moved in in 2019 and has had no service or maintenance On the unit. Now it is continuously blowing cold air and the thermostat is blank. Found a high limit switch was tripped near natural draft vent. Measured temperature rise and found that it was at 30°. Filter was heavily plugged up. Indoor blower motor has significant oil leakage. Circuit board has burn marks. Advised that safety inspection should be performed on system to verify heat exchanger is in safe running condition, client would like to hold off and get system replacement options instead. At this time unit Is operational but it is unknown how long it may run due to being past design life. Will be returning to go over system replacement options Tuesday at 5 PM on the porch or unit is operational