I have 2 issues to ask about please. 1) I keep getting a PO 0420 code, off and on, about every 6 months. I have replaced all of the O2 sensors and the cat. Converter but this still occurs. Any advice? 2) I have some occasionally white, "oily" smelling smoke coming out of the exhaust pipe for a few seconds after starting it, after the mortor has been off for a while. I checked the oil after a recent oil change, after about 10 days of driving and it was about 1/4th of a quart low. Should I be concerned? What could this be? (There does not appear to be any visible leaks anywhere).

White smoke is often a sign of coolant being burned. Did the engine ever overheat? If so, the aluminum head may have warped, causing the head gasket to leak. Have each cylinder's compression tested and a leak down test performed.

The engine has never overheated and the white smoke has more of an oily smell than a coolant smell.