F8 displayed, furnace will not ignite, fan may not start

Safety first: Disconnect power. Do not blow into pressure switches with your mouth; use low‑pressure air only if needed.

• Step 1 – Check combustion fan: With the cover removed, try to spin the combustion fan wheel by hand. It should rotate freely. If stiff or seized, the fan motor is likely faulty.
• Step 2 – Inspect pressure tubes: Locate the silicone tubes between the fan/venturi and the air pressure switch. Remove them and check for water, dust, or splits. Clean or replace as needed.
• Step 3 – Verify switch operation: With a multimeter, check continuity across the pressure switch contacts while the fan is commanded on (this test is best done by a technician). The contacts should close when sufficient airflow is present.
• Step 4 – Check flue and air intake: Ensure the concentric flue terminal is not blocked by ice, leaves, or nests. A blocked flue will prevent the pressure switch from closing.
• Step 5 – Replace faulty components: If the fan does not run or the pressure switch does not change state even with good airflow, replace the defective Protherm combustion fan or pressure switch.