Oxygen sensor issues

[bluefever00]

My dad has a 08 Yaris sedan. He recently had the exhaust manifold replaced. The first oxygen sensor kept setting off the check engine light immediately. He had it replaced with an ultra-power brand one from rockauto. Immediately after it started feeling like it was cutting out almost, but the check engine light didn't come on. They thought it might be the secondary sensor being older that it might be slower. That one was replaced with an ultra- power brand one. It cuts out somewhat less but the check engine light still hasn't come on. The car never acted like this before, it had an exhaust leak, but no check engine light and plenty of power for a car like it. Now it's kind of a dog under load and cuts out. Any suggestions?