Decided to fixup my faithful 02 Limited with 130K due to long neglected standard maintenance. I was getting the pre-cat fail codes on/off and then alway on. Once i put new plugs/wires/gaskets on it the CEL cleared, been a couple weeks now. I also replaced the plenum gasket, perhaps also related if a leak existed. I didn't use the OEM Denso plugs, I used Bosch 'Fusion' plugs that have a 4 tip arrangement and are Platinum+Iridium.

Whatever algorithm they use to check the front vs back O2 sensors is sensitive to a clean burn, so try a major tuneup first before spending the $$$ on pre-cats (in CA about $900 *each*).

On to find my mysterious oil leak. Front/back cam seals, valve cover gasket, and plug seals are all new, engine is mostly clean and degreased after timing belt change plus the above. I'm guessing it's around the oil filter bracket etc and blowing back into exhaust? No visible leak, no drips, idle for an hour no smoke, but drive for 20 mins and it smells of oil smoke on the passenger side of engine.

????