I was called to a home to repair a unit that was not running in fair condition. Upon arrival, I found that the unit was not producing any heat. I checked the thermostat and found that it was set to "off." I reset the thermostat and tested the unit again. This time, the unit produced heat, but the temperature was not reaching the desired level. I checked the air filter and found that it was dirty. I cleaned the air filter and tested the unit again. This time, the unit produced the desired temperature. I also checked the gas line and found that it was not leaking. I tested the unit again and found that it was running in fair condition.