The fan on my forced AC/ Heat will not turn off. Even with the thermmostat set to off and the fan to auto it keeps running. The only way I can get it to stop is to kill power to the whole unit. I have a digital thermostat that I installed about 4 years ago. Everything was working fine until a couple days ago. Any ideas?

Is this an older furnace? Does your a/c work?

Yes... I think about 15-20 years old. The ac and heat both work. Since I am using the AC now THe unit itself will turn off but the fan stays on.

Could be the fan relay in the air handler

You can see if it is your relay by taking the air handler cover off and you will see a square device in there with 4 or 6 wires turn your a/c on and let it run then turn it to off if fan is still running try tapping that box with the butt of a screwdriver if that makes it stop you know for sure it is your relay.

Relay can also be on circuit board. Remove fan wire from it and see if the fan stops.

In addition to the above suggestions, if you have an older furnace then you might have a fan limit switch that is keeping the fan running, if you have a standing pilot lite, it may be warming up the fan limit switch and keeping it hot. The fan limit is adjustable, they are located in the center of the furnace, usually have a removable cover to expose a round dial with arrows and temperature numbers on dial. There will be three arrows, the lowest one is the temperature off setting ( around 105 degrees) and the next will be the fan on temperature setting ( around 135 ) the other setting is the limit ( around 200 ) Only the low temperature setting do we want to adjust, If it is set at 100 move it up to 105, usually a small increase is enough to make it shut down if this is the problem (hold dial while adjusting arrow). This information ONLY applies to older furnaces, Good luck. Mike