Check enginge light codes

I recently had my engine relplaced in a lexus. However soon after having the engine replaced my engine light came back on again. I took it back to the delarship and they are saying nothing is wrong with the car that the codes are picking up something to do with the O2 oxygen sensors and they reset the light. Now after a couple days the light is back on again. What could be causing this? Im thinking about trading the car in. Is it a minor fix or is it a big problem that I need fixed?