Furnace lights briefly then shuts down; flame appears but is lost within a few seconds and the unit locks out.

Safety first: Isolate electrical power and close the gas valve before removing panels. Work only when the furnace is cool.

• Clean flame sensor: On many Sime furnaces the ignition and flame sensing are combined in one electrode. Remove the electrode and gently clean the metal rod with fine abrasive pad to remove oxidation. Do not sand the ceramic.
• Check grounding: Ensure the burner assembly and gas valve manifold are firmly grounded to the chassis. Tighten any loose screws or ground straps. Inspect the earth wire connection to the main control board.
• Inspect gas pressure stability: If other gas appliances fluctuate or you suspect low pressure, contact your gas supplier. Do not attempt to adjust the gas valve without proper instruments.
• Check wiring harness: Inspect the electrode cable and connectors to the Sime control board for damage, moisture, or corrosion. Reseat connectors firmly.
• Reset and test: Restore gas and power, reset the furnace, and observe operation. If the flame still drops out, the gas valve coil or control board may be defective and should be tested by a qualified technician.