If the car is running fine and no other indicators such as overheating are showing up, I would disconnect and then reconnect the battery(I believe this will reset the system) and see when or if the check engine light comes back on. If it comes on during idleing, then I would investigate a possible low oil pressure problem(oil pump or wrong viscosity oil?) or ignition problem(which should result in a rough idle I would think. Or, could it be something like your O2 sensor(has it ever been replaced?) Of course, it may not even come back on at all(remember-this is a Volvo with it's own quirky personality!). OR, you could just take it in for a diagnosis of the computer codes provided you trust your Volvo dealer. Good luck and please know that I am not a mechanic and don't even play one on TV.