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...

I am cooking noodles right now. You will get lashed with one :-)

REGULAR gas and full throttle???? It is a wonder the ECU didn't just shut down! Use 89 octane for normal driving and under 5000rpm, use 91 when you are ready to blow out the carbon.
Now, fill the tank with high octane to bring up the level of the 87 junk. You saved less than $1.80 and caused your self a lot of angst. Don't do that again.

Pull all 5 spark plugs and clean them off with a wire brush and check the gaps.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

Good to know. I take it by the curtness of your response that you believe my code could very well have been caused by open throttle with 87. Very interesting...

When the knock sensors detect noise, two things happen: The timing is retarded and fuel flow increased to cool off the cylinder.
My theory is that both of these things happened at over 5000rpm and at full throttle. The ECU retarded timing AND the fuel flow increased, causing raw gasoline to get to the CAT. The front and rear O2 sensors sensed the workload and the apparent CAT inefficiency and threw the code.

You should be OK. Just remember, 87Octane will lower your mpg even at normal driving range. 89octane works OK, but not over 5K rpm. 91 octane is what your engine wants for performance.

I tend to use 89octane on the flat Midwest Interstates and 91octane around the suburbs where I am always accellerating.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

Besides avoiding the CEL, do you think you make up the price difference going from 87 to 89 or 91 in increased gas mileage? I ask because I tried the 87 to 91 switch in both my 1980 245 and my 96 850, and was never able to measure any verifiable increase in mpg; and I fastidiously record my mileage.

My 1998 suffered terribly on 87octane, mpg down 2-3. The only difference between 89 and 91octane is the high end rpms with boost.
At 10 cents more per gallon, why hurt the engine?

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)