Mercedes A Klasse Hybrid Engine Trouble

Sounds like you've got a classic rich mixture issue! When you say mixture preparation, have you noticed any black smoke from the exhaust or a strong fuel smell? A defective fuel injection valve or injector is a prime suspect, as you suggested. But it could also be a faulty oxygen sensor feeding incorrect data to the engine management system. I would also check the spark plugs. Are they fouled?

No smoke or fuel smell that I've noticed. I haven't checked the spark plugs myself, but could a problem with the intake manifold also cause this?

While a leak in the intake manifold could cause some running issues, it's less likely to be the sole cause of a rich mixture. A faulty injector is a far more common culprit, especially with those symptoms. To be safe, I'd suggest getting it checked out at a workshop with diagnostic tools. They can pinpoint the issue pretty quickly.

Thanks for the advice! I took it to a workshop, and you were right – it was a defective fuel injector. The repair cost me 515€. Runs perfectly now.