Okay short version furnace is running fine then smoke alarm goes off near cold air return and we smell smoke in that area. I go to the other end of house to the furnace and hear it running but its throwing a code (4 blinks) limit switch so I unplugged it. Yes I called the fire dept. but thy found nothing only that the temp near the fan blower motor was 147.

OK so what is the model number and brand. Then what is the question?

If the question is what was the smoke and smell about it was probably the blower motor going out.

Like a high-quality jigsaw puzzle... all the pieces fit together.

Okay it's a coleman/evcon model # BGM07516AX its 11 years old I just replaced the gas valve and have cleaned the flame sensor three to four times. I removed the blower motor/squirrel cage and it spins freely by hand also the motor itself was not hot to the touch after the furnace threw a code. Would you say that considering all this it has to be a bad board? I might add that I was running without a furnace filter as I had cleaned it and was letting it dry out. Figures as it's the coldest night of the winter so far!

Could be... inspect both sides of board for evidence of burning and/or solder joint failure.

How right you are sir it was indeed the circuit board in fact my bro-inlaw found more than 1 burnt spot behind the board itself and a nice sized black burnt area on the sheet metal behind it also. Its not fixable so I guess we have to find another board. Thanks for your spot on advise.

If you noticed any evidence of arcing on the board mounting plate, make sure the new one is mounted correctly to prevent contact between the plate and the back of the board.