If it's a factory LH 2.4 system, you should have the diagnostic box on the LH side of the engine bay, near the LH strut tower. It's not fancy like a hand-held OBD code reader. The diagnostic box is just a LED and a jumper, but it does the job.
See the 700 / 900 FAQ for how the read and clear codes with it.
https://www.volvoclub.org.uk/faq/EngineOBDCodes.html