all you can do is what i do
explain to the customer that you will check to see if anything you have done would have caused this to happen but if it is another fault thats unrelated then it will be chargeable. I normally say i will not charge for the visit and do it as an extension of the original job
there will probaly be 3 boards. 2 power board (or more) and a control board. cant see them all going
i would check terminal block firstl bet some clown has not checked the links are tight
Dave