The problem these days is there are a lot of trades people whether it be gas engineers, electricians, or other service engineers that simply can't fault find. They don't seem to have the ability to test equipment and draw logical conclusions from the results they find. So they change this part and change that part until it works, that for me is not effective fault finding. Anyone with a little mechanical/electrical knowledge could eventually fix anything if they are given every single part of that piece of kit and told to just replace them until it works. That's fine until it is at the expense/inconvenience of the customer who as in this case has to put up with it. I've seen it time and time again with engineers I come across with my job. In a lot of cases it's lack of training but in this case it's probably due to the fact the decent engineers can earn more money working for themselves so you're left with the merely acceptable ones unfortunately!!!
