Vito: Poor Performance Due to Oil Issues

Hello,

On Saturday, a friend is coming by in his Vito with a 4-cylinder diesel engine. He has a problem where one of the cylinders is burning oil (due to a faulty head gasket).
Since the repair, it hasn't been running properly (it's very sluggish and has almost no power).

He had already taken it to the workshop, and they told him there was no fault detected (and charged him €100 for that).
Since then, it has been consuming less fuel.
If I didn't know that he was at the workshop right now, I would suspect that the mass airflow sensor (MAF) is faulty.

Since I'm currently ruling out standard electrical faults, I'm focusing on the catalytic converter. It could have been damaged by the oil, causing it to be partially blocked.
The question is whether this would result in lower consumption? (Logically speaking, it should consume more, right?)

Or, what other aspects might be relevant to the overall problem?
I won't receive it until Saturday, so I can't test anything beforehand and I just want to gather some ideas. Of course, I will also check the LD (likely referring to a specific feature or component).

Thank you in advance!

Is the problem still ongoing, or has it been resolved?

I understand it differently.
"If nothing can get out, then nothing can get in." (Cat is partially blocked.)
LLM sees lower throughput; control unit injects less fuel - or is that too simplistic?

Unfortunately, the person who was supposed to come is sick and couldn't make it, so not much progress has been made in the meantime.


I also strongly suspect it's a problem with the cat. (Unfortunately, I can't test it.)

I will post any new information here as it becomes available.