Hello everybody,

this is my first post, but since I decided to buy an S3 8L (APY, 209hp) I've already spent a significant amount of time reading through this forum and the S4 wiki.
I really thought I'm well prepared, I have all the necessary software, KKL Cable, Zeitronix Wideband O2 Sensor (not installed yet), boost gauge, AFR gauge, ordered an FIS Control unit (which might still take a while to arrive) and even wrote my first "Stage 0.1" tune two weeks before actually having the car.

Now to the problem: when test driving I was fairly sure that somebody accidentally or intentionally disconnected the tube from the N75 valve to the wastegate pressure cell. Boost was uncontrollably rising up to 2 bar when requesting some higher loads. Also all those small hoses and tubes on top and around the engine were a complete mess. The guy I bought it from just fitted a new stock K04 turbo, it was quite clear from the beginning why the old one failed.
So after getting the car that was the first thing. I pressure tested the intake system and cleaned up all the auxiliary hoses. It's all back to normal now. To my surprise, all the valves and hoses were stock and properly connected. Then I adjusted the pre-tension on the pressure cell, which looked to be a bit too much. With the N75 connector unplugged I now get 0.4-0.5bar of boost.
With the boost control valve connected I still get boost spikes up to 2bar when not being careful with the throttle.
So next logical step was to read the ECU bin and see what has been done to it.

Getting the ECU connected with NefMoto and reading the basic ECU info was no issue at all. When trying to read it, at first I got the message
"Disconnecting because no response was received for the Tester Present message."
After some more hours of reading the forum, I knew that I would have to put 12V on PIN121, if I would be using a bench setup. But I'm not. So another tip was to pull the dash fuse. Did that, played a bit with the Baudrate, and the error went away. Just to be replaced with the next one:
"Immobilizer not authenticated"
More reading told me that this happens when ECUs get switched out to different cars. To get around it I could open the ECU and get it into boot mode to disable IMMO, or apparently it's enough to power up the ECU with the right instrument cluster paired. The fact that this doesn't solve the problem lets me believe that the ECU has already been swapped out before?

Can anyone help me check if the ECU is stock or not? I'm not even trying to get any wild tunes on it (yet), for now I would be happy just to be sure to have a stock tune.
Do I really have to open the ECU and put it on a bench setup for that?
Also I've read that ME7check would not work with my ECU. Is that correct?
Can I somehow "force flash" an original file to the ECU, or reset it to factory settings in any other way?
If there's a tune on it I have no intention to keep it. Would still be interesting to have a look into it, but if that's not possible I'm fine with that.

Maybe unrelated: My blow off sound is really loud. I like it a lot, but it should really only be this loud with the N249 disconnected. Is there any way to check N75 and N249 are working properly?

Thanks you for any hints and suggestions!

Ok thanks I've now invested in a bench harness. Can't wait to have a look into that weird bin of mine..

APY turbos have a tendency to do wastegate creep, I've seen it on a few cars with brand new OEM VAG turbos fitted to them and started scratching my head on more than a few occasions...