After filling up the tank of my XC60 T5 with cheap fuel the check engine ligth lit up in orange.
The garage thinks that it can be ralted to an issue with the fuel.
I fill up with better fuel but my ODBC app car scanner is not able to delete the old DTC codes. So the check engine light is also stil lit.
How can I reset this light in the most easy way? Pull a fuse and put it back or something?

 

The only way to "reset" is to erase the code(s) with a compatible scanner.

What codes do you have? And if a fault continues when you try to erase it it will stay. So it's possible your scan tool is erasing the code, but OBD finds a problem immediately and won't erase it.

"Cheap" fuel won't make your check engine light come on.

 

Well the light went on after filling up with cheap fuel...

Code P0171 system too lean, bank 1

 

I managed to reset the check engine light.
When driving highway or other roads it stays off. As long as I drive no problem. Also longer trips are ok.
However when slowing down to a stop when the revs reach idle the light turns on again
What does this mean. Is the idle setting distorted by my last fuel fill with the bad fuel?

 

Further analysis:

When fully filling up the tank and resetting the error/light the error/light remains off for appr. 150 kms of driving (also city traffic).
After that:

The same issue as described before. As long as I am driving no issue, but when coming to a stop for 2 or 3 times the light/error appears again.

Any idea in what area of the system to look for?

 

An air leak between the air mass meter and the intake manifold could cause a leak condition and possible set that code.

What year, model, engine do you have? (yes I saw it was a T5 but 2020 T5 is very different than a 2013 T5)

 

2018 T5

But if that is the case you mention why is it only happening when coming to a stop and with a full tank I can drive appr. 150 kms without the issue at all?

 

Here's some suggestions from Vida as to why you keep getting a 171 code