right for the three people who have the same problem there is only one of four possibilities firstly and the most common is the fan is not removing the burnt gases quick enough causing the boiler to reach temperature to quickly to check this remove the front combustion cover if it stays on thats your problem secondly change the thermistors thirdly check the air pressure switch(very unusual) and last but not least sometimes the plug which goes on the thermistor is to slack not a good contact
ricky
December 2007