B Klasse Check Engine Light and Exhaust Leak

I had a similar issue on my B Klasse. A leak upstream of the catalyst can definitely throw off your emissions system readings. The ECU uses those readings to adjust fuel trims, and if it's constantly compensating for unmetered air, your fuel efficiency will suffer. Plus, the catalyst itself might not be reaching optimal operating temperature due to the exhaust escaping prematurely. Have you had the exhaust system pressure tested? That’s what I did to find my leak.

Could you elaborate on the nature of the exhaust leak you experienced? Specifically, where was the location of the leak, and what were the consequential symptoms beyond the check engine light?

My leak was at the flex pipe, a common failure point apparently. I didn’t notice any performance issues initially, just the light and eventually a slight increase in fuel consumption. But ignoring it could have damaged the cat converter. I would take it to a workshop to have them check it over properly.

A thorough inspection revealed an exhaust leak proximal to the NOx catalyst. Repairing the leak, at a cost of 265€, resolved the error code and restored optimal emissions system functionality.