Greetings all,
298,000 on the odometer of my '04 XC70 Cross Country. Several seconds after I arrive at a traffic light, I get a slight stumble for a second or two while idling. Not much. Maybe 50-75 rpm dip. Check engine light on. Code shows 02 sensor/cat issue. Any comments?
Will add some orher minor issues unrelated to drivetrain in another post.

 

which code exactly are you seeing? If its a P0420 code that's saying the rear sensor is seeing too much 02 compared to the front (pointing to a failing cat). There are other codes which will indicate a fuel trim issue - the O2 sensor may be saying its out of range, meaning it can't compensate enough but that doesn't say the sensor is bad. Root cause of fuel trim issues need to be triaged, ie could be an intake air leak or a torn vacuum line (most common), a dirty MAF or a contaminated sensor (is the car burning oil or consuming coolant?). If you can post up which CELs are being presented, that will help give you a diagnostic plan. I would suggest starting with the easy stuff like inspecting the vacuum lines, spraying some carb cleaner on the intake tubing/joints etc to see if there's an air leak, even consider having a smoke test done if nothing obvious shows up. You can try cleaning the MAF (using a product like CRC MAF Cleaner, not carb cleaner), and inspect the intake air tubing for oil or dirt (oil could be coming from the PCV system). If you have an OBD2 reader, you may also want to try and capture some run time data on the fuel trim.