This can be a serious headache in some situations. We've had several jobs that had days of diagnostics and research. They turned out to pay only 4 or 5 hours as we couldn't see charging these people thousands of dollars. These have made some of our best repeat customers though. On the other hand we've got a regular dealer who sends us only the tough jobs and nearly every time, we're able to bill every hour spent to get the repair done.
There's only been a couple of jobs we had to turn away and tell the customer the car just isn't worth the can of worms this one repair is going to open up.
I tend to estimate high. Assume the worst case scenario and add a disclosure to the estimate that we cannot guarantee. If the customer still insists to move forward we'll take the job on. This has really been our only saving grace but it took those two worst case scenarios to come to this method of handling diagnostics.
Anymore we have more headache in this area from the people who think they are going to come in for free diagnostics, that we'll tell them what 's wrong and they can then attempt DIY repairs. I send them to AutoZone.
~Heather