Code 12, furnace cycles on and off rapidly or fails to modulate correctly

Safety: Turn off power before touching pipework; surfaces may be hot.

• Check system valves: Ensure all isolation valves on the flow and return pipes near the Vokera furnace are fully open.
• Inspect system filter: If a magnetic or mesh filter is installed on the return line, isolate and clean it according to manufacturer instructions.
• Bleed air: Remove air from radiators or air handlers to improve circulation.
• Verify pump operation: Listen for the circulation pump running when there is a heat demand. If silent or excessively noisy, it may be failing and require replacement.