I have two central AC units. One of them will not shut off when the desired temps is reached, It will also not shut off when the switch is set to off. The only way to turn it off is with the breaker. I have changed thermostats, disconnected the thermostat wire to the unit, and even swapped the control board and still have the same issue. Both the blower and condenser keep running. When I turn off the breaker to the blower/furnace the condenser shuts off. Any ideas of what it could be?

Did you remove the stat wires on the board?

I did disconect the stat wire. Thought it may have been shorted (mice). No change.

Yea I was thinking rodent too. Post the model number and we can see what we can do.

I would remove one of the low voltages wires to the contactor in the condenser unit to see if it opened, breaking the high voltage circuit. If the contactor opens, I would then remove a low voltage wire at the air handler. If contactor opens you can then look for the relay that opens to break the connection to the contactor.