LMM and Cat Issues: Troubleshooting Guide

Hello,
"For the past three days, my Passat TDI 3B (5), manufactured in 7/97, with 92,000 km and 90 hp (AHU engine), hasn't been accelerating properly. It takes a long time to reach just 140 km/h on a flat surface. Before this, for a few weeks, it has been making rattling noises from the front part of the exhaust system. Yesterday, I tried the following:"
1. The LMM (Pierburg) was sprayed multiple times with brake cleaner, with no change in result. The grilles also make it difficult to spray accurately.
2. LMM connector disconnected, result: no change.
"Short-circuiting the LMM (lambda measurement module) connector with a diode, without modifying the EGR (exhaust gas recirculation) system, resulted in the engine running very poorly. It exhibited rough running, misfires, and excessive smoke production, particularly under partial load."
1. Question: Does this clearly identify the LMM as a source of error?
2. Question: Can the Pierburg mass airflow sensor (MAF sensor) be easily replaced with a Bosch MAF sensor? The connector probably won't fit, right? (Bosch MAF sensors are available on eBay, but I haven't found any Pierburg ones yet).

The rattling sound from the exhaust system is likely coming from a faulty catalytic converter. Since it's probably deteriorating further, it would be advisable to replace it. There's a possibility that the exhaust flow is already restricted. However, the fact that the car ran relatively well with a diode bridge connected to the mass airflow sensor (MAF) connector might argue against this. But if the catalytic converter isn't completely blocked now, it could become a problem at any time.
Now, my problem is:
In which direction will this data center be expanded, upwards or downwards?
Which other components should be disassembled beforehand?
It looks like the rest of the exhaust system will need to be removed.
It looks incredibly cramped up there. I have no idea how to loosen those four nuts at the top without disassembling other components (and which ones, at that). Okay, two of the nuts are easily accessible, but the other two...
Unfortunately, there is no detailed manual available for this vehicle, and the "How it's done" guide by Etzold doesn't offer any information either. I haven't found anything else yet.

What was my old Passat (model year 1981, 54 hp) so easy to repair .

I would appreciate it if you could give me some tips and advice.

Have a lovely Sunday and a good start to the week.
Claus.

Hello, based on the test you performed, your LMM is clearly defective.
You get the cat out from up there.

Hello,
You can get the Pierburg mass airflow sensor (MAF sensor) from VW as a replacement part for around €80. I would definitely have it replaced.
Regards,
Max.

Hello,
Since I received a few more answers to my question, I would like to provide a final report to the forum.
Since the cause of the error was unclear, I contacted A*U in desperation. Although this company, like others, sometimes receives negative reviews, I was surprised by the helpfulness of the team in Darmstadt. I read the error codes: the mass airflow sensor (MAF) was defective. They provided me with a MAF sensor for testing purposes, but the result was no change. Two days later, we removed the catalytic converter, which was loose and rattling inside the housing. Of course, it was blocking the exhaust pipe, and it had really become embedded. We replaced the catalytic converter, and everything was okay. I cleared the error codes, took it for a test drive, and read the codes again, and everything was okay.
As far as I recall, the cost included the catalytic converter itself and its installation, and it was around 390 euros.
There's nothing to complain about.

It's unfortunate that the catalytic converter only had a lifespan of approximately 90,000 km. Its mounting should have lasted longer.
Best regards, Claus.

Hello,
Thank you for the feedback!
Bye.
Max.