Q: My mercedes c300 says collision prevention assist plus inoperative

The warning light means there is an electrical malfunction in the collision avoidance system and that could involve sensors, wiring, the control computer and so forth. Anybody who fixes it (whether you, a Mechanic, etc.) has to have in hand a copy of the Factory Service Manual. In the manual is a "diagnostic decision tree" for this specific system and the procedure is to start at the top of that "tree" and work your way down until the problem is pinpointed. If you are lucky, the problem will be visible right at the top. If you are unlucky, you ’ll spend a couple or several hours and the problem will turn out to be the last item on the tree. Most problems fall somewhere in between. If you desire a mobile diagnostic, with the required repairs performed right at your location, please request an electrical component/circuit diagnostic and the responding certified mechanic will get this resolved right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.