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The circulating pump works fine, and both the hot water and central heating work fine if/when the gas fires up and stays lit, the problem is that it does not always light properly. The boiler goes through a rapid cycle, it sparking, the gas lights, then goes out immediately, then it sparks again, the gas lights and then goes out etc etc. It does this repeatedly until it either stays lit or the cut out button pops out. I thought it might be the flame sensor but I have changed that and there is no change.