1990 740 Not idling properly

[EyeLiveToFish]

Howdy fellow Volvo enthusiasts, I was lucky enough to receive a 1990 740 GL as my first car from my parents. Its been an amazing journey thus far however her check engine light is on, we took her to a local 5 star Volvo mech and he attempted to fix the light by hooking up a computer to her and checking for error codes. We received 3 codes, 2 of which he was able to fix and the light went off. But what do you know on the ride home, blink the check engine light pops on. The engine hasn't been idling properly as the steering wheel will shake when your sitting at red light and the engine doesn't sound right when she idles either. Any advice will be helpful, thank you - EyeLiveToFish