Sounds like they made an "educated" guess based on experience. Hopefully they talked to you before doing this and left the ball in your court. When I have to do this I explain to the customer why I would suggest said repairs and that this may not be the correct part to replace but there is NO WAY to know. We blame the shop yet there is NO WAY to tell where his problem lies. If the problem is consistant then it is a ton easier to diagnose. But the problem is when the truck magically starts, now what? Hmm, let the truck go and have a pissed off customer when it breaks down again or explain to the customer the situation and let him make the decision. I dont think the distributor was a bad guess and seems rather logical. We dont yell or ***** to the doctor when he misdiagnosis us, we just let him rip off our insurance and keep on billing us and putting us through physical therapy until the problem is fixed. But bottom line, I hope they will make it right with you. In this situation I would now just begin to charge you for parts, no diagnosis time or labor. But if the shop TOLD you this was for sure the problem then they are in fact semi-responsible.