The reason why the code came quicker with new O2'* is because you old ones were a little slower on the pick up.
The code is set becasue the O2 picks up and reports to the PCM that more fuel is needed. The PCM exceeds it'* expected thresholds and reports that it'* adding the extra fuel to you.
Check those vacuum lines on the front valve cover (L36 motor). I know this one from experience..
Were the cables corroded under the boot before you brushed them? I've seen and replaced my cables because the corrosion was down in the cable and the power difference because of it..set the code.
Last thing is..check the EGR tubes for cracks. Right at the base where it is bolted to the manifold. That'* the normal crack area.
All of these except the cables are vacuum leak related and will cause a P0171...