I bought my 99 V70 T5 5 spd on Saturday, successfully smogged it Tuesday, and this afternoon, the CEL came on, reading code P0422 only. I've read over the board, and understand that its a discrepancy between the O2 sensors, and could mean they need replacement, or the cat, or there's a vacuum leak, or some other problem causing the engine to run rich.

However, the light came on right after I absolutely floored it getting onto the freeway. After reading an older comment from Klaus about a rich mixture or a bad tank of gas also causing the code, I'm wondering if a situation like mine where 10 seconds of 5000 rpm driving after several hours of 2000 rpm conservative driving would send an influx of unburned fuel into my exhaust, triggering the code. Another thought; I filled the car with regular (87), and for all I know it's been running on premium (91) all it's life. Could that sudden switch in octane also cause a code like that to be thrown?

I've reset the code and I'll see if it comes back, but I'd love to discuss theories...