I had same code on a volvo c70, checked for vacuum leaks but wasn't any, ended up changing the air mass meter, and no more codes... Sometimes if an air mass gives the wrong signal, usually during part load, the oxygen sensor will notice that the fuel mixture is too lean and try to compensate for it, but if it is way off it can't and therefore will trigger the fault code.

If possible try to hook up a volt meter to the front oxygen sensor and check the readout during acceleration if it goes down and stays low. If it does there is really only 2 things that could be faulty, Air mass or low fual pressure/clogged filter.

Only bad thing is the Air Mass meter is kinda expensive to buy just to try it :) so if you know anyone that has a volvo with the same one on , you could try to swap it.

Bjorn Smeds / Swedish Motors LA