Hi Sarah.
You emailed this to Worcester Bosch didn't you ;) Worcester don't like to try and give tech advice to homeowners because they never know whether you'll try and attempt a repair yourself.
But for guidance's sake, it's easy enough to test a transformer, he doesn't need to replace it until he's identified whether it's failed or not.
It's a tough call this one, cos there are several components that could cause the fault - we could be looking at a sensor fault, or we could be looking at a heat exchanger problem (but that doesn't account for the blown boards).
How much have you paid your engineer? You may wish to consider calling a Worcester engineer out - they work on a flat fee (£210 unless you live in London) which includes all the parts and a 30 day recall period in case it goes wrong again just after they leave! It comes to a point where if you're paying £150 for a board, and £70 or so for a gas valve, that you have to wonder which way is most economical.
It's almost impossible to pin down the component that's causing this from the details you've given. Sorry.
Bunslinger
May 2009
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