Furnace shows E3 and will not start, fan may run briefly

Safety first: Turn off power and gas. Hot surfaces and flue gases can cause burns and poisoning. Do not operate the furnace with a known flue fault.

• Check flue terminal: From outside, ensure the concentric flue terminal is not blocked by snow, leaves, or debris. Clear any obstruction.
• Inspect air intake: Verify that the air intake path is clear and that no objects are stored too close to the furnace blocking ventilation.
• Check flue gas sensor connection: Open the combustion chamber cover and locate the flue gas NTC sensor on the flue collector. Ensure its plug is firmly connected and the cable is intact.
• Sensor resistance check: With power off, a technician can measure the NTC resistance and compare it to Remeha specifications at room temperature. Replace the sensor if out of range.
• Heat exchanger cleaning: If the flue path and sensor are fine, the heat exchanger may be fouled. This requires professional cleaning and inspection of the entire flue system.