[DW42]

It should have LH2.4 injection, in which case you can check the computer's diagnostic codes. Use this guide: https://www.volvoclub.org.uk/faq/EngineOBDCodes.html

I wonder if the oxygen sensor was disturbed during the exhaust repair. If it's not sending a signal to the ECU you can end up with the fuel mixture running too rich, fouling the spark plugs, then making the car hard to start and run poorly. This theory would be supported if you get codes 1-1-3 and/or 2-1-2.