If there are codes for both O2 sensors, like 435 and 436 then before doing anything I'd carefully look at your vacuum lines because it's rare that both would be bad at the same time. A vacuum leak trips both O2 sensor codes. If non-turbo, remove the plastic cover over the throttle body and look at the hoses going to the vacuum T there. If turbo, check the hose from the intake manifold, behind the PS pump and its outlet at a T just above the turbo air intake going into the large air intake tube. Those are the most common areas where I see cracked or deformed (from oil) vacuum lines.
Inexperienced techs will often misdiagnose those 2 codes together for both O2 sensors.