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. Vokèra split the code into two rows, and which one you have matters. A flue thermistor fault, or a fault with what they call the flue thermistor counter, is classified as temporary and will clear itself. A flue thermistor over temperature is Final: it stops the appliance until somebody resets it.

The reason the second one is treated that way is the flue itself, because plastic flue systems are rated for the temperatures a condensing appliance normally produces and sustained overtemperature is what they are not designed to handle. What the counter actually counts is not something Vokèra explain in the manuals we could reach; their instruction to the engineer is to check it at power on.