Boiler shows E12 and shuts down shortly after ignition, often with noisy combustion

Safety: Turn off power and gas before inspecting venting. Do not operate the boiler with a known flue fault; risk of carbon monoxide.

• Inspect exhaust vent: Check the entire accessible length of the exhaust vent for kinks, sags, or disconnections. Outside, ensure the vent terminal is clear of obstructions and not too close to windows or air intakes.
• Check intake/exhaust spacing: On concentric or side-by-side venting, verify that intake and exhaust are properly spaced to avoid flue gas recirculation.
• Examine flue sensor: Locate the Dettson flue temperature sensor on the flue outlet. Inspect wiring and mounting. A loose sensor can misread temperature.
• Professional combustion check: A technician should perform a combustion analysis, verify gas pressure, and adjust or repair as needed. If the flue sensor is defective, it should be replaced with an OEM Dettson part.