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The most likely cause for this is poor combustion and an incorrect fuel/air ratio. Your tech should perform a full combustion analysis and make adjustments accordingly. When the flame is too oxidizing or carburizing, it is considered a 'dirty flame' and will coat your flame sensor with deposits. Causes for this are either too little/too much gas pressure at the burners or too little/too much air flow through the burners. This should have been adjusted at the original installation and checked every couple years for proper adjustment.
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