Engine Performance problem
2003 Chevy Monte Carlo 6 cyl Front Wheel Drive Automatic 70000 miles
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i scanned the computer and i received code po404 which was something do with improper egr something i replaced the egr valve and i have the same problem there is also a whistling sound when i try to accellorate. any thoughts on what my problem might be?

it had a severe lack of power won't climb hills or shift properly it seems

check exhaust system
do a back pressure test on the exhaust system checking the catalytic converter flows

check for vacuum leaks
and MAF sensor