Does the unit actually fire just not stay lite... if so then the flame sensor is the culprit..sometimes you can pull it out and lightly sand it don't go crazy because to much sanding can really cause problems... if the unit does not light then what is the sequence it goes through before failing... this unit have a hot surface ignitor i am assumming (glow plug some call it) or is it spark ignition with a module box on it... either way the sequence of operation should be a call from the thermostat made for heat, the enducer draft motor comes on (small motor with a wheel on it) then the enducer draft motor pulls a negative pressure on the pressure switch (round switch with hose attached to it) make sure hose is not kinked or broken,also pull off and check for debri in hose and in holes where it connects to... after pressure switch closes it will send power to ignitor /or spark module once glowing the gas valve should open and allow gas to enter burner orfices and fire ... once unit has lighted then the flame sensor continues to measure milivolt and send bacl to board to keep gas valve open while call for heat... if the flame sensor is dirty or faulty it will not allow the gas valve to stay open. repost with more info ///watch unit where does it fail... most goodman units have a code schematic on the door of unit or inside... should tell you what the number of flashes means if it still is there let it fail and count the number of flashes and then see what code is for that failure.... good luck to you