Does anyone have problem with 1996 Mercedes Benz S500, failed smog test due to OBD and ped Emission?

I have a 1996 Mercedes Benz failed smong test due to OBD (On Board Diagnostic) and ped emission. We replaced the O2 sensor and still failed, no failure codes detected when plugged to the OBD2 handheld device. Does anyone ever experienced this problem? Please help!!!