Citroen DS4 Exhaust Smell Rough Running

A rough running engine, an exhaust leak, and a strong smell alongside the check engine light strongly suggests a problem within the exhaust system. While a faulty catalytic converter cannot be ruled out entirely, consider focusing on sensors first. Have you considered the lambda sensors? A defective sensor could easily cause the symptoms described, especially the rough running and incorrect exhaust readings, leading to the strong smell.

Thanks for the suggestion. I had considered the lambda sensor, but dismissed it quite early on. If it was the sensor, wouldn't I get a specific error code for that? I'm just trying to avoid shotgun troubleshooting and replacing parts unnecessarily.

While a dedicated error code for a lambda sensor malfunction is ideal, it's not always the case. A 'lazy' or out-of-range sensor can throw off the entire exhaust management system without triggering a specific sensor fault code. The ECU might misinterpret the readings and compensate incorrectly, leading to the symptoms you’re experiencing. Given your symptoms, investigating the lambda sensor thoroughly would be prudent. I would suggest a workshop, they can measure the values with a multimeter and can tell you for sure.

Thanks a lot, that gives me a much better direction to focus on. Took it to the shop and, as you suspected, turned out to be the lambda sensor. Replacing it sorted out the issue, total cost 375€.