Mercedes GLS NOx Issue and Exhaust Leak

I had something similar happen on my GLS. The exhaust system was the culprit. Car diagnostics showed high NOx emissions, but it was a small crack in an exhaust pipe throwing off readings. Double-check your exhaust system before assuming the NOx catalyst is failing. Any changes in engine sound or performance?

I haven't noticed engine sound or performance changes, just the check engine light and exhaust smell. When you say double-checking the exhaust system, what do you mean? Are there specific areas prone to leaks to inspect?

Double-checking means visually inspect all exhaust system joints and welds from the engine manifold back. Pay attention to flex pipes and where the exhaust connects to the catalytic converter and muffler. Small cracks are hard to spot without a good light, sometimes feel for escaping gas when the engine is cold. Given the exhaust smell, a leak is likely. With that mileage, a professional car diagnostics check at a workshop might pinpoint it.

I took it to a workshop, and it was an exhaust system leak, as you suspected. The NOx catalyst was fine. The total repair cost was 265€. Your experience guided me to the correct diagnosis and saved me from unnecessary repairs.