1998 Saturn SL2 Check engine light

Engine Mechanical problem
1998 Saturn SL2 4 cyl Automatic 109000 miles

During expensive repairs (new radiator, water pump, couple hoses, etc.) The car shop said these 2 codes were showing up because the check engine light is on: P0133-Oxygen circuit slow response bank 1 and P01133-Manufacture controlled fuel and air meter. They didn't urge us to have it checked out immediately like it was an urgent situation. So, what do these mean? The car seems to be running fine and they said it would cost $94 just to find out what is actually wrong. Are these codes generally a really bad thing or are they a more an indicator of a minor problem? Any info would be helpful. Thanks.