number one possible issue:
IF YOU HAVE AN EXHAUST LEAK BEFORE THE O2 you will have a poor running car and a SES code

Second:
Check that O2 wiring is not running under or to close to Distributor. The Distributor makes a signal that the O2 wires pick up and send false information to the ECU. The ECU will interrupt this as a run condition and cause the car to run ugly. SES code will show.
this includes any high draw wiring near O2 wires.