Water heater burner assembly and combustion chamber probably needs clean and maintenance.
I suggest calling sears service 1-800 469-4663 or plumber.
I'm on Sears/ American/ Rheem/ Reliance etc gas water heater websites most days, and read gas water heater manuals continually.http://www.sears.com/shc/s/search_10153_12605?keyword=kenmore+power+miser+9+40+gallon+gas+water+heater+gashttp://www.sears.com/shc/s/s_10153_12605_Appliances_Water+Heaters_Natural+Gas
Most people don't fix air conditioning without knowledge going in, but people think water heaters are simpler, because water heaters were simpler a few years ago.
Today, gas water heaters require testing equipment, and specialized tools, plus replacement parts.
Possible problems include: Improper fuel-air mixture, dirty air intake, venting problems, gas control valve problems, air in gas line, low gas pressure, dirty combustion parts, Water condensate dripping on burner, chemicals stored inside same room. Service technician will review all these things during troubleshoot, and you can watch technician and glean information for keeping water heater working longer than warranty.http://waterheatertimer.org/Replace-anode-rod.html
If repairs are going to cost more than $400, then install new gas water heater yourself same day:http://waterheatertimer.org/How-to-install-gas-water-heater.html
Upvote the help.
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