A word about diagnostics codes: all of those that begin with the letter P are proprietary. This means that, in general, P0171 on a Ford will not mean the same thing that it does on a Volvo. It *might* mean the same thing, but the P prefix should be considered a warning that it probably doesn't.

With that said, I've had a similar code from the same family of "fuel adaptation" codes. In my case, it required replacing the mass air sensor, under warranty at the time. It's a common enough problem on these cars such that I've often considered that, if it ever happens again, that I would try cleaning the sensor wire with throttle body cleaner. That may or may not work, but if it does, it will save me a few hundred dollars for a new sensor.

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Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html