The results you wrote of certainly doesn't describe a bad turbo - just the opposite. I'm not positive but I really don't think a Turbo would directly cause a "check engine" -- really only if it passed a lot of oil and clogged the cat - an indirect effect. My own experience with a water cooled turbo was a '92 940T -- bought it at 168K with original turbo - wrecked while parked after putting another 15k on it - still working perfectly.
A balky AMM sounds more likely to me. -- Dave