Unit not heating. Checked system and board was throwing 4 blink fault code which is main limit open. System is over heating. Checked for cracked heat exchanger. Read 0ppm on supply air. Checked for dirty filter. Checked supply air and return air temperature. Everything seemed to be working. Basked of age and the fact there were some rust spots near secondary heat exchanger it likely the secondary heat exchanger is clogged causing excessive heat buildup in unit. Will email over estimates for new furnace to Customer
