Check engine light still on after replace O2 senors and ECU and EGR

I had a friend replace the O2 sensor under the exhaust because thats what the emissions code said and that check engine light stayed on so i replaced the ECU unit and EGR valve and took it back to emissions after driving 300 miles.  highway...street, etc.  Emissions came back with the same O2 sensor code.  Can anyone please help with any suggestions???