Upon arrival unit was on but not running. Turned thermostat up. The unit started to light but a couple seconds after the flame came on it put the flame out. Inspected the flame sensor and found it was very corroded and starting to get thin in some places causing it to not sense the flame. Went to a supply house to get a new flame sensor. Removed the old flame sensor and installed the new one. Also found the air filter was plugged and covered in dog hair. Replaced the air filter with a company supplied one. Started and tested the unit. Unit is working properly.