Furnace shows 05, burner does not start, fan may run briefly then stop

Safety: Isolate power before opening the casing. Do not modify flue components unless you are qualified; incorrect flue work is dangerous.

Steps:

• 1. Inspect flue terminal: Ensure the terminal is clear of obstructions and not covered by snow, vegetation, or objects.
• 2. Check condensate trap: A blocked condensate trap can cause water to back up into the fan and pressure tubes. Locate the condensate trap, place a container underneath, and carefully remove and clean it. Flush with clean water and reassemble, ensuring correct seals.
• 3. Inspect pressure tubes: With power off, remove the small silicone tubes from the air pressure switch and fan. Check for water, dirt, or splits. Blow through gently to clear any blockage and refit securely.
• 4. Listen for fan speed: On demand, the fan should ramp up before the pressure switch closes. If the fan is slow or noisy, it may not generate enough pressure.
• 5. Technician tasks: A technician can test the air pressure switch with a manometer and multimeter, and replace it if it does not switch at the specified pressure. They will also verify correct flue configuration and fan performance.