Vokèra [A09] fault codewhat it means and how to fix it

A condensing boiler should send comparatively cool gas up the flue, because extracting that heat into the water is the entire basis of its efficiency. A09 means the flue thermistor has either measured a temperature well outside normal, or has failed in a way the board can detect. Where the reading is genuine, the cause is nearly always that heat is not transferring properly into the water — scale or sludge lining the heat exchanger, or circulation too slow to carry heat away — so it leaves through the flue instead. The reason the appliance locks out rather than logging a warning is the flue itself: plastic flue systems are rated for the temperatures a condensing appliance normally produces and sustained overtemperature is exactly what they are not designed to handle. Vokèra also use a flue thermistor counter on some models, which locks the appliance out after a set number of overtemperature events rather than on a single one.