If I have a good idea what the problem is I tell the customer that, but I won't tell them what I think it is. My usual statement is something like "If I tell you what I think it might be and then it turns out to be something different you'll say "but you said..." so let me make some tests and confirm what I think it is, book it in." If they do great, if they don't and they wanted a free telephone diagnosis then you were never going to get the job anyway. I'm sure I've helped loads of people go to the local spares place to fix their own cars. I had one guy say after about 3 phone calls over the same problem "I'll have to send you a Christmas card this year" I think I said "pop a £20 note in it then". No card, no money.

Shaun