The pilot light is lit (good news), however when I turn the thermostat to "hot" and then turn the black knob from "pilot" to "on", the main burner doesn't light.

I kept it on for approximately 20 seconds but then turned it off because I don't want gas to build up. I'm thinking either thermocouple, the inside-the-tank temperature sensor going to the thermostat. The flame sensor may be dirty and needs to be cleaned; that said I can't see the flame sensor through the viewing port. Does it take longer than 20 seconds for the main burner to light?


link to AO smith water heater parts site showing flame sensor, so perhaps my unit (model FSG 40242) has one?

Igniter 9005256015 Order now for same day shipping. 365 day return policy. RepairClinic.com

photo of my burner and pilot light, I don't see the flame sensor but perhaps I'm blind:

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Can you hear the gas coming out of the main burner? Turn your thermostat all the way up to see if it makes a difference. Thermocouple should be ok if the pilot is staying lit. I'm guessing you aren't waiting long enough for the gas valve to open and send gas to the main burner.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I'm in the process of taking the thermocouple, pilot and burner out for a thorough cleaning. Since it appears the thermocouple is white in color, I think I'm going to replace it. The nuts holding the thermocouple and pilot light to the main valve came out easily.

The 19MM nut holding the supply line to the burner is STUCK. I'm thinking of using PB Blaster spray to loosen it......of course PB Blaster spray is "flammable". With the gas supply valve turned off, is there any chance using PB Blaster spray will pose a problem?

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I use Kroil applied with a 22 gauge needle on a syringe for stuff like that. A few drops beats PBB hands down.

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You're a daisy if you do.
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84 Euro 240D 4spd. 220.5k sold
04 Honda Element AWD
1985 F150 XLT 4x4, 351W with 270k miles, hay hauler
1997 Suzuki Sidekick 4x4
1993 Toyota 4wd Pickup 226K and counting

Husky you can find lots of information if you visit Google search page , put your question in the searchbox.I repaired my heater this way.

If the pilot is staying lit, your thermocouple should be fine. You need to listen to see if gas is being released to the main burner. Try tapping on the gas valve if it isn't.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

OK, the pilot light produced a yellow flame so I purchased a new thermocouple and installed. I also took some compressed air and cleaned out the pilot light itself from both ends. I then reinstalled the burner, pilot light and thermocouple into the gas hot water heater. Started the pilot light and the pilot now burns BLUE!!! For those who don't know, yellow (like in the picture above) is NOT GOOD because it indicates a blockage somewhere in the pilot light. Yellow doesn't get hot enough either....what we want is a BLUE flame, NOT yellow.

The burner started right up and I should have hot water within a few hours. Special thanks to everyone who stepped up and assisted with this issue. I don't know that much about gas hot water heaters so all of the help was most appreciated!!

Yellow typically means it's not getting enough air.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I did not read the other answers. However, in the past I have had some sort of build up inside of the Valve Body plug a passage and a similar issue with the little regulator that was gunked up.

Had to take it apart and clean it. However, that was when they were only mechanical inside.

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84 300D, 82 Volvo 244Gl Diesel