Hi I have a 2004.5 S40, 2.4I that has been showing a bank1 too rich situation code PO172. I've cleaned the MAF meter, ran a tank full of hightest fuel through the car and added a injector system cleaner and no luck. Every time I clear the code it comes back a day or two later. Any suggestions? Should I start changing the O2 sensors, which one first and where is the location of the sensor you suggest changing. Thanks!