Will getting my spark plugs change turn the check engine light off

Spark plugs will generally cause the engine to run roughly and not the symptom you provided. The ideal approach and the one that will prevent you from replacing unneeded parts will be to check the diagnostic codes and perform a diagnosis. That said, these S40's will exhibit this symptom some of these reasons: broken air hose, defective idle air valve, or a fuel delivery issue. If you can get the code and post it that will help. If you need a shop, try on of our Certified shops near you: http://repairpal.com/volvo-within-10-miles-of-brentwood-california-94513