F3 displayed, furnace does not start, fan may run briefly

Safety first: Disconnect power and close the gas valve. Wait for the furnace to cool.

• Step 1 – Listen for ignition: When the unit tries to start (before you open it), you should normally hear a rapid ticking (spark) and then gas ignition. If there is no spark sound, suspect the ignition system.
• Step 2 – Inspect ignition electrode: Remove the burner cover and locate the ignition electrode. Check for cracked ceramic, bent rod, or heavy deposits. Clean the metal tip with fine abrasive and ensure the gap and position match the Protherm service diagram.
• Step 3 – Check ignition cable: Inspect the high‑voltage cable from the ignition transformer to the electrode. Look for burn marks, cuts, or loose connections. Reseat both ends firmly.
• Step 4 – Verify gas supply: Confirm that the manual gas shut‑off is open. If other gas appliances work but the furnace does not ignite, the Protherm gas valve coil or control signal may be faulty.
• Step 5 – Technician verification: A technician should test the ignition transformer output, measure voltage at the gas valve during ignition, and verify correct gas pressure. Replace the ignition transformer, electrode, or gas valve as required.