Ok...so now I have a turbo for my 2003 Audi A4 1.8t. But the mechanic who checked it out said it could also be a valve located before
the cat. He said that there is a spring on the valve that could be broken. My question is, is there any way to determine if it is the turbo or the valve without tearing the turbo out? The car does build boost and hold's the boost in the system...there are no leaks. The engine light is on and the code is "Supercharger/turbocharger under boost"

Thanks for any help you can provide.

P.s. I checked all over this forum for an answer...but to no avail, I don't want to waste anyone's time.

its the wastegate you refering too.
there is no spring there, but there is a wastegate regulator that is connected to the wastegate via a rod. Check the wastegate regulator by unpluging it u will let the turbo spool all the way, kinda dangerous, dont let it spool all the way.

The only vavle that has a spring is your Diverter valve and if thats boken it a 30 part and you can install it your self, if you want to upgrade that part check online for 007 forged ones. The original diverter valve is un serviceable and when the diafram is broken then you won't hold boost and you will lose all of the boost pressure. There are plenty of discussions about this all over the forum.

Mac

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I'd guess DV as well. When mine went on my S4 it set a similar code "turbo charge pressure out of range" or something similar

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06 S4 Neuspeed Exhaust, B&M Shifter, 20" MHT Paragons, PSS9's, Tinted Everything....
04 GSXR1000 very fast...