backgroud- I have a 1993 model Comfortmaker furance model#GU100a016ain that runs for about 15-30 miuntes then goes out and furnace indicator light signals that the high limit switch is opened and then system locks itself out from trying to restart, all while never reaching the set temperature. Sometimes it works for serveal days then as soon as the outside temp. gets cold, the problem returns. I have had a HVAC techican look at it several times and all seem to come up with a different reason why it keeps going off and on, but none can fix the problem. I have replaced the control module board,flame ignitor,high limit switch,sereval thermostats=now using reg. mercury stat.,checked and cleaned coils, & replaced the low voltage wiring, in which the first problem came from,were the system was saying not calling for heat, when it really was showing calling for heat.