Upon arrival furnace was flashing 3 times indicating it was off on a limit switch code. Checked the limit switch and it was good. Checked the air filter and found it was plugged. With a plugged air filter the heat exchanger of the furnace could not fully cool down causing it to fail with the limit switch code. Replaced the air filter with a customer supplied air filter. Cycled power to the unit to clear the code and started the unit. Unit is now working properly