My initial replies to such questions are very vague. I generally begin by saying "it could be a number of things, It depends. I'll know better when I come out an look at it." After I'm at their location I can get a lot more specific.
I look at my service call fee as the price for my technical knowledge, and the travel to their location. If after I give them the diagnosis, they want to order their own part, and install it or not, I figure they've paid me for the consultation. If they do the rest, and botch it, they can pay me again.
On the other hand, if it is an error code call that just requires a reset (dishwashers are common for this) I explain to them how to reset it, and how to run a diagnostic. This still fits into the half hour I provide for diagnosis, and gives them some value for the fee I charge. I find that customers like a bit of explanation, and it increases your expertise in their eyes. Besides, they'll still call you back based upon your service and the trust you've established with them.