How to troubleshoot flame sensor errors in a gas furnace
When getting a flame sensor error always make sure the furnace is properly grounded before you do anything else. An ungrounded furnace will pull a little ground off of the gas line but this is not a good ground source and the furnace will work when it feels like it and drive you up a wall. If the ground is good then cleaning the sensor rod with a little steel wool or fine grit sandpaper will usually fix it.
I had a customer once that had a heater problem. Every fall their previous A/C guy had to come out and replace the flame sensor. This went on for years and finally they got fed up and called me instead. I knew right away the problem once I saw the old 2-prong socket in the attic. Luckily the water heater was close by so I ran a new ground wire to a ground clamp on the cold water line and the furnace fired right up. The following fall I got a "thank you" card from this customer because it was the first time in years the furnace worked right the first cold spell.
Source: AMANA GUID090EA50
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