gas furnac burner

[dvdgspncr]

Having a problem with an older gas furnace burner lighting but going out within 45 seconds. Have changed limit switched which ohmed good and the dial type limit where the stem is inside the burner housing. I have continuous 24 volts to gas valve during the whole process as well as when the burner goes out. Any sugesstions or help would be appreciated. Not much hair left to pull out. Thanks

[how]

Brand,model & year of furnace always helps. Older gas furnace means different things to different folks.
Are you talking about a straight pilot light with a thermocouple going out within 45 * Are you talking about an ignition system with a flame sensor locking *
If a gas valve that has a continuous 24V shuts down then either the gas valve is faulty, the gas supply is inadequate, or a primary safety control that is connected to the gas valve has shut it down.