this relates to a problem i'm having with my gas furnace. i took a power hit and ever since then the blower on my furnace continually runs when i have the thermostate set at off and fan on auto. i repalced the thermostate no help. I did notice the fan runs slightly faster when i switch it from auto to on. i'm thinking it might be the circuit board. Am i on the right track and is there a way to confirm that. It's a carrier natural gas furnace built around 1989. model 518cxx042000maca. i looked on line for the circuit board and found one for about $70. part # hh84aa011. What's your advice....:confused:

The circuit board has the relay for the blower on it. If the rest of the furnace is working OK then i would feel the board is at fault.

I would try this before I went for a new board.

Take the cover off of the board if it has one, I think you will see two large relays. With the furnace off but with the fan blowing use a plastic handle screw driver to tap the side of each one of the relays. The points could be stuck and dion this might knock them loose.

I think others on here would agree with me that it is a shame that these are not plug in relays.

I agree with the relay thing. I guess if the relays were replaceable like the plug in's I have then the board will cost more. Usually if a relay sticks it means the contacts are burnt and once burnt the only way to fix them is with a very fine contract dresser file. I still have the files in my retired service bag of tools. LOL That is why I usually recommend just replacing the board since most folks will not know how to get the relay cover without damage and then you have to know how to dress the relay contacts back to the original dimple shape. The relays today do not have the extra materials on the contacts like they used to years ago and when you hit them with the file its all over.