My girl's IS has a lack of power up in the higher rpm range.. After 4k I feel the car starting to struggle. I've done a tune up and it helped, but it doesn't feel like an IS should. Car runs ok in the lower rpm range but I can tell it's not like it use to be. The car idles fine too, so I'm wondering if anyone has had this issue? Any ideas are welcomed since with no cel it's anyone's guess at this point.

 

Are you sure there is no check engine light - make sure it turns on when you turn the key on but the engine isn't running...

Aside from that, your issue will definitely be hard to diagnose... When is the last time the plugs were changed? How many miles are on the car?

 

Check plugs for wear, also go ahead and clean the MAF and TB with correct cleaner

 

What parts did you replace with your "tune up"?

 

Found the issue, it was the third cat. Thanks for your replies.