Furnace attempts to ignite but fails; burner does not light and unit locks out after several tries.

Safety first: Turn off electrical power to the furnace and close the gas isolation valve before opening the casing. Wait for all hot surfaces to cool.

• Check gas supply: Verify that the main gas cock to the Sime furnace is open and that other gas appliances in the home work normally. If gas supply is suspect, contact your gas provider.
• Inspect ignition electrode: Remove the burner access panel. Locate the ignition/flame sensing electrode assembly near the main burner. Check for cracks in the ceramic, damaged cable, or heavy oxidation on the tip. Clean the metal tip gently with fine emery cloth and wipe with a dry cloth.
• Verify electrode gap and position: Consult the Sime furnace service label or manual for the correct gap and position relative to the burner (typically 3–4 mm). Adjust carefully using needle‑nose pliers without stressing the ceramic insulator.
• Check burner and combustion chamber: Inspect the burner for dust, rust flakes, or debris. Vacuum gently and avoid deforming burner ports. Make sure the air intake and flue are not blocked.
• Reconnect and test: Reassemble the burner cover, open the gas valve, restore power, and reset the furnace according to the Sime instructions (usually by pressing the reset button on the control panel). Observe ignition through the sight glass if available.
• If ignition still fails: The ignition transformer or control board flame circuit may be defective. At this point, contact a qualified Sime service technician for live‑gas and live‑voltage diagnostics.