my heater has gone off and its not igniting, you can hear the ignition trying to come on but its not lighting or staying on. I have had 3 engineers here but they have not managed to solve the problem, how can the problem be fixed
You'll need a good engineer as removal of the combustion chamber is required.
They'll need to check for an adequate spark gap, ensure that the electrodes are generating a spark and that the gas valve itself is opening allowing a small amount of gas to the burner ready for ignition.
I'd seriously consider hiring a reputable gas engineer for the job in question.
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