Exhaust Fan Runs With No Power at All!

There might be an error code on your electronic control board. It's located in the lower half of the furnace. Sometimes there's a little plastic window that you can look through or you might have to take the cover off.

There'll be an LED on the board that will blink in a special sequence and give you a code number. There should be a label inside the cover or inside the wall of the furnace that tells you how to decipher the code.

If this problem just started (the furance hasn't always operated like this), then it's likely an open high limit switch.

If it's a manually resettable limit, you can try resetting it yourself. If it's an automatic reset device, you'll probably have to replace it.