Apr stage 1+ Underboost code - AudiForums.com

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Old 10-08-2017, 10:40 AM
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Join Date: Oct 2017
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Default Apr stage 1+ Underboost code

Hey all,

so I'm having a problem since I got apr stage 1+ with Underboost code
Here are the details.
2004 Audi a4 Quattro, 140xxx miles
apr Stage 1+
full 3" Turbo back exhaust with a test pipe
forge diverter valve
stock k03 turbo and ic
new silicone and billet pcv
late 04 AMB engine
Tt 225 injectors

So the problem started with apr stage 1+. Before apr, I was running stock tune on like 9lbs and did not have this code. The car would push 9lbs to red line. After apr, of course optimal power band is still 4k to 5500 rpms, however, the car now pushes 22lbs but only to about 4700 ~ 5000 rpms. After that the boost rapidly tapers down to about 10lbs. Car stared throwing a cel for Underboost. Is this just due to the k03? I was under the impression that apr stage 1 + was optimized for the k03. Any help would be appreciated. Thank you.
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