I've got an old old Mueller Climatrol model W125-145SUR, and the fan won't stop. I've set all the thermostat settings to off; I even tried removing the thermostat device off the wall (and hence disconnecting the circuit). The fan just stays on the whole time. Any ideas?

Thanks!

Most likely the high limit in the furnace is fualty, there may be a switch on limit that you can set to on or auto, if it is set on auto, then your limit is most likey at fault. Check filters, if they are extremely dirty, they may have caused furnace to over heat to many times, and ruined the limit, Can you post a pic of the inside of furnace?

Here's a pic of your possible limit