here's volvo's information:

Diagnostic trouble code (DTC) information

Condition

The control module receives information from the heated oxygen sensor (HO2S) about fuel / air mixture. If the fuel / air mixture deviates from λ=1, the short-term fuel trim will compensate for this by adjusting the injection interval so that λ=1 is achieved. When the short-term fuel trim makes an adjustment, the long term fuel trim (λ) will also be adjusted. A diagnostic trouble code (DTC) is stored when fuel trim has reached an upper limit (lean limit) or a lower limit (rich limit).


Substitute value
Fuel trim disabled.


Possible source

Rich limit:
-Contact resistance in the terminals.
-High fuel pressure.
-Leaking injectors.
-Contaminated oil.
-Defective heated oxygen sensor (HO2S).


Lean limit:
-Contact resistance in the terminals.
-Low fuel pressure.
-Air leakage in the intake system
-Leakage in the exhaust system
-Defective heated oxygen sensor (HO2S).


Fault symptom[s]
-Malfunction indicator lamp (MIL) lit.


P0171 is rich, so you have to worry about those. Sorry, I believe that means its not a leak. Fuel pressure is easy to check. 150k is decent enough life on your 02 sensor (though I'm not saying go out and replace it right now.. many times some ), so that wouldn't surprise me. The oil thing is easy enough to eliminate. If your due there's that, if you just changed, there's that its unlikely, and if its the middle, pull out enough from the cover to verify color and smell. If it smells sweet or like gasoline you've got problems.

Now first off, has it come up more than once?