F1 displayed, furnace locked out, no ignition attempt

Safety first: Isolate electrical power and close the gas shut‑off valve before working inside the furnace. Allow the unit to cool.

• Step 1 – Inspect flame electrode: Locate the flame ionization electrode near the main burner. Remove the mounting screw and gently pull it out. Check for cracks in the ceramic insulator and heavy oxidation on the metal rod.
• Step 2 – Clean the electrode: Use fine emery cloth or a non‑abrasive scouring pad to clean the metal rod. Do not sand the ceramic. Wipe off dust with a dry cloth. Reinstall, ensuring the tip is correctly positioned in the burner flame path as per the service manual.
• Step 3 – Check wiring and ground: Follow the flame sensor cable back to the PCB. Ensure the connector is tight and the cable is not damaged or pinched. Verify that the burner assembly has a solid ground connection to the chassis (tighten ground screws).
• Step 4 – Reset and test: Restore power and gas, then reset the furnace. Observe ignition. If the burner lights but F1 reappears, the flame signal may be too weak.
• Step 5 – PCB diagnosis: If a known‑good electrode and wiring are installed and grounding is correct, the flame detection circuit on the main PCB may be defective. This requires board replacement by a qualified technician.