Furnace LED shows code 43 (pressure switch calibration fault or limit circuit fault depending on model).

Safety first: Turn off power before working on pressure switches or wiring.

• Verify model and switch rating: Check the Carrier model number and compare the installed pressure switch part number to the furnace parts list or label. Using a non-OEM or incorrect rating switch can cause this code.
• Inspect wiring: Ensure the pressure switch wires are on the correct terminals as shown on the Carrier wiring diagram inside the blower door.
• Check tubing and ports: Confirm that the tubing is connected to the correct port on the inducer or collector box and that ports are not plugged.
• Reset power: After corrections, turn power off for 30 seconds, then back on and call for heat to see if the code clears.

If the correct switch is installed and the code persists, the pressure switch or control board may require replacement by a professional.