I would agree with you that the rear O2 sensor should have taken care of the P0136 code.
The P0171 code has a faulty mass air flow (MAF) sensor among its possible causes, and that's a common failure point among our cars. It's also a bit pricey, at about $160 from FCP Groton, but at least it's easy to replace.
You'd do well to look into All-Data for diagnostic help. See http://www.jimrothe.com/volvo/backyard_mechanics_resources.html.
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Jim Rothe, '99 S70 T5M, http://www.jimrothe.com/volvo/index.html
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