My buddy has a 1999 camry LE (US market) with the 4 cylinder. It throws a check engine light which shows o2 sensor is bad and is causing it to run lean, we replaced the sensor and now it runs worse. It runs okay as long as its cold but as soon as it warms up it runs rough. Sometimes it runs fine and others it stumbles and hesitates real bad. Anybody got any ideas?

Is it ok going down the highway, but just idles badly?