heil model nulk075dg04 short cycling

[jakkmeyer]

hey everybody. i am new to the forum and really need some help my heil gas furnace is short cycling it runs for about three minutes then the heat kicks out , then about 20 seconds later it starts back up again.i have bypassed the stat and it still does it. i have replaced the filter cleaned the probe and removed the blower fan to make sure secondary heat exchanger was not clogged. all seems to be clean. can the problem be the fan and limit probe . i dont know what else it could be . could it be the heat exchanger the exhaust is not plugged what else is the problem. the probe is gonna cost almost 200 bucks to replace.

[how]

You say that the furnace runs for about 3 minutes. Are you saying that the burners, inducer motor and the fan blower are running for 3 minutes and then the main burners drop * Three minutes is a pretty fast limit for a furnace with for an operating fan, clean filters and a/c coil, and no ducting restrictions. Do you have an electrical meter to determine where the voltage stops on the safety limit, roll out switches and inducer's pressure switch circuit that shuts down the gas valve. Do you know if the gas valve is still getting 24V when the gas valve shuts * Can you see a change in the flame around the flame sensor just before it shut's * Measuring the temp rise of the furnace and comparing it to the temp rise on the furnace plate will tell you if the limit is supposed to be shutting down the burners. Make sure any ground wires are securely connected to bare metal. Is there a flashing diagnostic led on your board that corresponds to a fault legend on the furnace *