Replace the thermocouple. when you do make sure electricity and gas to the system is off. It's cheap to try but will likely not buy you much time. When you do replace the coupler, drain the hot water tank. The will belp flush and crap out of it. Also clean stuff off and out as best you clean with a soft wire brush. It will cost you under 20.00 in most cases to try. Regardless if it works or not, prepare to replace it soon. I just replaced mine. It was 11 years old under same situation I bought an additional 2 years by doing this. Now ive upgraded to a 40 gallon . If you dont already have a 40 gallon, I don't recommend the upgrade if you have teens in the house. The more hot water available, the more they use it :/