The check engine light came on and the engine was idling rough. I assumed it needed new spark plugs because it calls for it at 50k miles. After replacing the plugs, the problem was still there. I took it to a private shop which diagnosed it as a fuel injector. Again, this did not fix the problem. Finally, I took the car to the Mercedes dealer and they informed me of low compression on cylinder #3. Mercedes is an embarrassment if they force their customers to pay for a new engine after 50k miles. My wife's Lexus is running strong at 178k miles.

Update from Aug 25, 2021: Mercedes diagnosed the engine with a cracked piston. They charged almost $7000 for repairs.