Furnace indicates a power supply or mains frequency error; unit may not start or may reset intermittently.

Safety first: Disconnect power before opening the electrical compartment.

• Check supply source: Ensure the furnace is connected to a stable mains supply, not an undersized generator or inverter that cannot maintain correct voltage and frequency.
• Inspect power terminals: Open the electrical junction box on the Sime furnace and check that live, neutral, and earth connections are tight and free of discoloration.
• Measure voltage (electrician): Have a qualified electrician verify that the supply voltage is within the range specified on the furnace rating plate.
• If supply is stable but error persists: The internal power supply section of the control board may be failing and the board should be replaced by a technician.