The small temperature thermocouple which has a geen plug on it with two yellow wires has failed or has been tightened up too much because it may have been leaking. It has a fibre washer on it which is crap, and when it is tightened up too much it twists the internall parts so it cant latch properly. Everything works on the boiler but it just wont ignite, its a bugger.
monty, May 2010