C Klasse Check Engine Light and Exhaust Leak

Had a similar issue with my old BMW. Check engine light, the whole shebang. Turned out to be a faulty NOx sensor throwing off the air-fuel ratio. Since you mention exhaust leaking, that could definitely point to a problem in the exhaust system caused by the sensor malfunction. NOx sensors are part of the pollution control system, so if it's not reading correctly, it can mess with everything. Is the leak near the sensor?

Air-fuel ratio? Pollution control? You're speaking Greek to me! Where exactly is this NOx sensor located and is it easy to check the cable or plug connection myself?

Okay, so the NOx sensor is usually somewhere in the exhaust system, often after the catalytic converter. It measures the amount of nitrogen oxides in the exhaust. The air-fuel ratio is the balance of air and fuel your engine burns; the sensor helps keep that optimized for cleaner emissions. If the connection is visibly loose or corroded, that could be your issue. If you aren't comfortable, a workshop can check. They quoted you 95€ for the sensor replacement?

Turns out the cable to the NOx sensor was indeed damaged, just as you suspected. It was causing the exhaust leak and triggering the check engine light. The workshop confirmed it and fixed the connection. Saved me a ton of money on a new sensor!