What was the most difficult technical problem you've ever had to diagnose? What instruments or diagnostic devices did you use?
When responding to a question like this it is often useful to use the C.A.R. or Challenge, Action, Result strategy. For example, talk about the challenge you were confronted with, the actions you took and the results you achieved. The interviewer will appreciate knowing that you are a problem solver who has a record for achieving results. You'll want to demonstrate that you have a technical grasp of automotive mechanics by talking specifics, but don't lose sight of the big picture. The interviewer wants the details but what's most important is that you have the needed technical aptitude and should be able to pick up on any new technologies as they're introduced.