Mercedes CLA Diesel Engine Running Lean

That sounds rough! A lean running condition can be a pain. Reduced power and jerking aren't fun. You mentioned a few possible causes, which is good. Have you checked for any obvious vacuum leaks? Sometimes a cracked hose can throw off the whole system. And the check engine light - have you pulled the code? Knowing the code will really narrow down what's going on. It could be the oxygen sensor giving bad readings, messing with the fuel mixture. Or even the mass air flow sensor underreporting airflow. Though you're right, a faulty injector could definitely be to blame. Is it misfiring badly at idle?

Is it feasible that a faulty oxygen sensor could trigger a code indicative of a lean condition, even if the actual problem lies with a fuel injector? Also, how can vacuum leaks be identified efficiently?

Absolutely, a bad oxygen sensor can definitely throw a lean code even if the injector is the real culprit! The O2 sensor is just reporting what it sees in the exhaust, and if the mixture is off, it'll complain, regardless of the reason it's off. As for vacuum leaks... you can try listening for a hissing sound near the intake manifold. Sometimes spraying carb cleaner lightly around vacuum lines will cause the engine to stumble if it sucks some in. But honestly, with those kind of symptoms, and especially with it being a diesel, I'd get it to a workshop. Messing with the fuel system on a modern diesel without proper equipment can cause more harm than good.

I appreciate your advice. Following your recommendation, I took the vehicle to a workshop. The diagnosis confirmed a defective fuel injector, and replacing it resolved the lean running condition. The total repair cost was 515€. Thank you again for your assistance.