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Yes when ever I had this it was the plate with black flakes inside it. Even running it under the tap and hammering it with the heal fo my hand forcing water through in both directions, flakes would come out and then it worked fine. Never got a call back on it either. No harm to change the thermisters but usually not necessary. On Alphas you best change both as it montiors primary also during hot water. Pump proving is by thermister, it monitors the pair and knows from relative change and rate of change whether circulation is adequate. Shuts down if not.
apply for Vaillant groupservice would be my advice mate.
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