it could be various things, first check the seal on the combustion door (the one that covers the burner) it could need replacing, the door it's self might be mis-shapened, the bolts might not be holding it firmly, when you take the combustion door off & look at the heat exchanger in the four corners are rubber seals (round black) they might need cutting back as they could be holding the door off of the door seal or the fan overheat stat might be faulty, check the burner isn't damaged (very easy as it's only fabric) hope this helps
Keith Gillyon Plumbing & Heating
December 2008
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