My A/C cools below the set temperature. I turned the Thermostat off and the furnace kept running. I had to turn off the furnace at the breaker. Trying to figure out what might be the problem.

Try disconnecting the red or yellow wire to the thermostat at the furnace. If that stops the A/c from working, the problem is in the thermostat or its wiring.

If the blower continues to run, but not the outside unit. Likely its relay is bad. If both the blower and outside unit continue to run, likely the board is bad.