2006 Volvo S40 code P0172

Your code means "system too rich, bank one".
Which indicates a possible leaking injector or fuel pressure regulator.

Check for vacuum leaks, check fuel pessue and make sure oil is not contaminated with fuel. If all that is good then remove the throttle body and clean it out. You will also need to reset the fuel trim adaptations. You can usually do that by unplugging the ECM connectors on the side of the air cleaner box. Would also recommend you take it in to Volvo and see if there is a software update for the Engine Control Module. If one is avalable have it done. That will recalibrate some parameters and how it looks at the fuel trims.

Yup