The gas valve is most likely defective, that is really the only thing that can cause this problem. they are cheap and easy to replace. REMEMBER SHUT THE GAS OFF THE WATER HEATER AND THE HOUSE TO BE SURE OF NO FIRE OR EXPLOSION BEFORE ATTEMPTING ANY REPAIRS..
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The gas valve is most likely defective, that is really the only thing that can cause this problem. they are cheap and easy to replace. REMEMBER SHUT THE GAS OFF THE WATER HEATER AND THE HOUSE TO BE SURE OF NO FIRE OR EXPLOSION BEFORE ATTEMPTING ANY REPAIRS..