There is no thermocouple in direct spark water heaters. They use an electrode & flame rectification to sense the flame. A lot of issues with this type of system results from impurities in the air (such as dust, moisture, etc.) being burned Onto the flame "rod" a simple cleaning with an alcohol type eyeglass lens cloth should work ok. DO NOT use sandpaper or a file! Most flame rods/sensors have very tiny groves along the length giving more surface area to detect the flame. sanding can leave residue in these groves that will burn onto the rod.
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