L67 running too lean

[Bigbird3]

Hey I know this car probably is in your guys domain but my 2002 grand prix GTP its running slightly lean and my mechanic suggested that it may be because if the exhaust being too free flowing. The exhaust was done less than a year ago and the check engine light just came on though. The exhaust is stock but no cat and flowmaster 40 series delta flow (for decreased interior resonance which is bs it still drones). What do you guys think I should do and any suggestions for the exhaust because it doesn't sound very good anyways so I kinda want to change it.
Thanks in advance,