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Hello, you will need a multi meter with a Micro Farad scale on it to measure the current flow from the end of the flame sensor wire to the printed circuit card where it attaches.
ooops... Micro Amps. Not micro farad
We will be looking for 2 to 5 micro amps.
If you have that current flow from the flame sensor wire to the card and are still getting a flame failure error, then the printed circuit card is most likely bad.
If you do not get a 2 to 5 micro amp current flow to the card, then it could be the fault of the card or the a bad ground in the flame sensor circuit...in that case a 'flame simulator' is needed if you want to prove the card is bad or not. $45 made by Johnson Controls.
It puts a simulated 5 micro amp signal to the card, if the no flame error continues then the card is bad for sure.
If the furnace now works, then there is a faulty ground at the burner head, due to rust on the burners or a problem with the furnace grounding.
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