Quick update: I took the equivalent of your paint thinner advise by adding catacleaner to the fuel tank. Check light was off significantly longer - instead of every 30 miles, it come back after 120 miles. I followed up with a can of BG44k and the light is still off after 160 miles. Please tell me if...

Boosted5cyl and Jimmy , thanks very much for very helpful leads!! I'll try changing both O2 sensors in Bank 2, as well as adding thinner to the fuel ... I understand it is about a gallon to a half empty tank and then running the car for about 150 miles at highway speed. By the way, bank 2 in a XC90...

I got myself a OBDII tool ... The only codes I get are PO430 (in fact I always get them twice, for some reason). I clear the PO430 codes and they come back after about 30 miles of driving. The OBDII tool gives me the following FREEZE data: Trouble code: PO430 Fuel Sys Sta B1: CL, using HO2S Fuel Sys...

About a month ago, I got a P0420 code and a mechanic talked me into changing Bank 1 cat, which I did with OEM parts. Check engine light went back on in a day after the change. Then the mechanic said that he had overlooked changing the two O2 sensors in bank 1, and did so. Check engine light stayed o...