Furnace shows an ionization or flame signal error even though the burner appears to be lit.

Safety first: Isolate power and gas before accessing internal components.

• Clean flame sensor: Remove the flame sensing electrode from the burner. Clean the metal rod with fine abrasive pad to remove soot or oxidation. Reinstall ensuring it is correctly positioned in the flame.
• Check grounding and polarity: Verify that the furnace is connected to a properly grounded outlet and that live/neutral are not reversed. An electrician may be required to correct wiring.
• Inspect wiring harness: Check the ionization cable from the electrode to the Sime control board for damage or loose connections. Reseat connectors.
• Measure flame current (advanced): A technician can measure flame rectification current to confirm if the control board is reading the flame correctly. If the signal is good but the board still faults, the board may need replacement.