Kia Rough Idle and Power Loss

my car has had a loss of power, running sluggish, that sort of thing.

so i had the spark plugs replaced, and had to replace my radiator hoses just due to a minor leak.

after about driving for 20 minutes after having replaced the plugs, my check engine light came back on.

now when im sitting at a stop light or the car is idleing, it is really rough, the car actually seems to shake quite a bit. after you accelerate it seems to be fine, but it will cut out of power a lil bit on hills, it actually seems like it has trouble getting into 3rd gear or higher.

i was just trying to see what else i should look into, i had the shop run it through the computer, and they said it seemed fine.
after i left not 10 minutes later it started and the check engine light came back on.

the only thing i can think of really at this point is maybe the oxygen sensor?

besides having like (advanced auto parts) run a diagnostic, what else should i try?

the mechanic seemed to just blow me off, after i just paid for them to do absolutely nothing.

please any advise would be helpfull.



thanks