forced to go to the workshop

Good morning.

I'll probably have to visit the Audi service after work today, as my TDI seems to have gone into limp mode this morning (suddenly limited to just under 110 km/h). The car has been running poorly before (maximum speed of about 160 km/h), and I was planning to look into it properly during a quiet weekend. So, I'm going to have the error codes read first, and I wanted to ask you if you have any tips before I let them try to sell me something I don't need. What else should I pay attention to?
This is the first time my 80s-era 1Z (old model) with 345,000 km has given me so much trouble.

Thanks & Regards,
Lucas.

Hello Lucas,

Your error description is not very informative.
Without reading the error codes, I can't give you any helpful advice.

Best regards, Rainer.

Hello Rainer,

I was also looking for a tip regarding the diagnostic process at the workshop, as some pretty interesting things can sometimes happen.
Of course, it's too early to speculate, especially since I haven't had a chance to check the most common sources of problems (fuel filter, hoses, etc.) yet. I was just worried about the sudden drop in performance (possibly a failsafe mode), although I suspect it might be something 'more serious.'
Okay, but I'll tell you more about it when I'm feeling more informed tonight.

Regards,
Lucas.

Hello Lucas,

As always, in the workshop:
- make sure you get a complete readout of the error memory.
- Consider having a measurement run performed. However, you should accompany me during that run. Make sure that the necessary prerequisites are met (see error database).
- Don't let the mechanics pressure you into immediately replacing everything.

Hello again,
Embarrassing but true: the diesel filter was (only) clogged.
Furthermore, yesterday, two VW stores and one ATU were not open.
to read the error codes from the device on-site (due to time constraints).
and the latter because of a missing adapter). Also good money.
saved!
One might forgive him for his scaremongering...

Regards,
Lucas.