Mercedes SLC Check Engine Light and NOx Catalyst Error

It appears you've encountered a predicament familiar to some. I recall a similar situation with my 2016 BMW 328i with a gasoline engine a few years back. The NOx catalyst efficiency warning illuminated the dashboard, accompanied by a faint but persistent exhaust odor. The prevailing theory at the time centered around the exhaust system integrity. Have you conducted a thorough inspection for any visible cracks or perforations along the exhaust pipes, particularly near the catalytic converter and the NOx catalyst? Also, are you experiencing any fluctuations in fuel economy or unusual engine behavior beyond the illuminated check engine light?

Regarding the exhaust inspection, I haven't had the chance to get under the car myself yet. I was hoping it was something simple before having to crawl around the garage floor. When you had the issue with your BMW, did you also suspect the NOx catalyst itself, or were you fairly certain it was the exhaust leak from the start? How did you pinpoint the exact location of the leak? Is there a trick to locating it that I should know?

Initially, the NOx catalyst was a suspect, but the exhaust leak presented a more plausible explanation given the symptoms. The leak was pinpointed using a smoke test, wherein smoke is introduced into the exhaust system to reveal any escape points. It's a fairly common diagnostic procedure. Given your symptoms and the potential impact of exhaust leaks on NOx emissions control, it would be prudent to consult with a qualified automotive repair technician for a comprehensive evaluation. They can perform the necessary diagnostics to ascertain the source of the problem and provide an accurate estimate for the automotive repair. A NOx catalyst issue can be expensive, so ruling out the exhaust leak first is wise.

I took it to a workshop and you were spot on! There was a small crack in the exhaust manifold causing the leak, which was throwing off the readings from the sensors and triggering the NOx catalyst error. The final bill was 265€, which is way better than a new catalytic converter! I'm glad it was a simple automotive repair, and now I am back to enjoying the open road!