Am a hvac student working on a older 150 highboy gas furnace Burner goes out within 1 minute before blower comes on. Have changed limit on heater as well as one that has the dial on it, thermocoupler, fan relay,scratched my head , crossed my fingers and still same problem. Gas valve is all thats left that I'm aware of that it can be. Anyone with something else to try? Also I have 24 volts at gas valve before ,during and even once burner goes out.