Furnace LED flashes code 43; indicates pressure switch or venting problem during high-fire or low-fire operation on two-stage models.

Safety first: Turn off power before accessing internal components.

• Verify vent sizing: Compare vent pipe size and length to the installation label on the furnace. Excessive length or too many elbows can cause draft issues.
• Inspect both pressure switches: On two-stage Day & Night furnaces, there may be separate switches for low and high fire. Ensure all tubing and wires are correctly connected per the wiring diagram.
• Check outdoor terminations: Confirm both intake and exhaust are clear and properly spaced.
• Reset and test: After inspection, reset power and call for heat, observing whether the furnace can transition between stages without fault.

Stage-specific pressure problems require measurement with a manometer. A technician should verify switch setpoints and inducer performance.