I just bought my '91 240 yesterday and it had a check engine light on when I bought it. I pulled codes today and got 113 and 121, which correspond to fuel injectors and the afm. I'm hoping not to spend a ton of cash on fixing these problems, so I'm going to try running a good fuel cleaner through and use maf cleaner on the afm. The engine seems to run well enough so I don't really think the injectors are fully shot, I'm hoping a good clean out will fix it. Anybody have any experience with these codes and the best plan of attack?

 

Oh, also for the first minute or so driving sometimes, the "service" light will light up but it goes away.

 

The service engine light is inconsequential; it is a reminder to change the oil. It can be reset.
You will find that trouble codes on Volvos often are ambiguous at best. While it will certainly not hurt to run some injector cleaner (doubtful they're shot) what you need to have is a digital multi-meter and at the very least, a Haynes manual. Checking the voltage range on the AMM will let you know if it is acceptable. I'd start with the O2 sensor as it is the lone sentinel that tells your ecu how it is doing. A lazy or dead O2 sensor will cause issues elsewhere in the system. Trust me on this...get a manual and a volt meter...check the voltage variation at the O2 sensor. It's best to check it also at the ecu but that involves removing kick panels and such. Locally, my Volvo buddies are sick of me preaching about oxygen sensors...they're particularly important on Volvos. Forget emissions for a moment...the O2 is critical for the entire system to operate correctly.
Also...there is really no way of knowing how old the codes are...it's possible the issue was fixed some time ago. Be sure to clear them and see what comes back.