So did it work when you brought the truck to them and not work when you left, or did it fail later the same day? If it's the former you might have a logical leg to stand on, if the latter, I don't see how you can prove it was the mechanic's fault, nor do I think you'll do well in court. Assuming the scenario where the sensor failed later in the day, and given the fact that these things fail with regularity, I don't see how you can say with any certainty that your mechanic caused the problem.
If it's thirty bucks and five minutes, just fix the thing and get on with your life. Why waste time on something as trivial as this? If you're uncomfortable with these folks, just don't use them again.