I have been asked to repair a Ariston Micro combi MFFI for a letting agent I do a load of work for (I try to avoid Aristons like the plague) I have tried working my way round the boiler with my multi meter but have drawn a blank. I have bundles of H/W and the boiler chugs along quite merrily with no problems, when the boler fires in C/H mode it is fine for approx 3-5 mins but then shuts down. (makes me think a Stat).There is an amber light which is a "Fume sensor" and this goes out when the hot water is being used but continues to flash when the C/H is used before it cuts out. I have had the flue elbow off as well to check that the small bypass tube is blocked, but this model does not have one. Anybody got any ideas.??