I have a 30+ yr old lennox furnace and the fan will not shut off. I just replaced the fan limit switch and yet that didn't fix the problem. The thermostat is set to auto and so is the limit switch. Some of the troubleshooting I have done is setting the limit switch to manual and leaving the thermostat at auto, not sure if that would even work but I've tried it and it didn't. I'm sure I wired it correctly. The only thing I'm not sure of is the low voltage jumper tab on the limit switch. Its an expensive part and I didn't want to ruin it so I haven't broken the jumper. I tried reading the wiring diagram, to compare to other websites but that didn't help out too much. Is the jumper tab my problem or is it a relay or something? I've read through the other posts to find an answer but nothing so far. Is there anything I can do to rule out the limit switch as the problem?

When the thermostat is turned to the OFF position does the fan still run?

In AC mode, the thermostat controls the fan.
In heat mode the heater (limit switch) controls the fan.

At 30 yrs, I'd suspect this unit might have a fan center and/or the relay that controls the fan stuck on.

The fan still runs at OFF, I'll dig a little deeper and try to find a relay

This is a "fan center" ZETTLER Controls Inc. - HVAC/R Products > Product Line (http://www.zettlercontrols.com/products/product.asp?pp=fancenter) It was used in older furnaces.