Navien furnace displays E012, unit will not run, sometimes after heavy rain or snow

Safety: Turn off power and gas. Do not operate the furnace with a suspected flue blockage due to risk of carbon monoxide.

• Check exterior vent termination: Go outside and inspect the exhaust termination. Remove any snow, ice, leaves, or insect nests. Ensure the termination clearances meet Navien specifications and are not blocked by shrubs or structures.
• Inspect vent piping inside: Follow the PVC vent from the furnace to the outside. Look for low spots where condensate can collect, loose joints, or signs of collapse. Correct slopes to ensure condensate flows back to the unit or to an approved drain, and re-glue or secure any loose joints.
• Check condensate trap and drain: A clogged condensate trap can cause water to back up into the flue path. Remove and flush the trap, then verify the drain line is clear and properly pitched.
• Verify vent length and configuration: Compare the installed vent length and number of elbows to the Navien installation manual limits. Excessive length or elbows can cause E012. If the system exceeds limits, have a professional redesign the venting.
• Reset and test: After clearing and correcting venting, restore power and gas, reset the unit, and monitor operation. If E012 returns, the flue pressure sensor or main PCB may be faulty and should be tested by a technician.