engine management lights not going out?

the two stop lights on instrument panel were coming on ,the car went in for diagnosis and the fault code showed up as rear lambda sensor needing replaced which was done. The following day the lights came on again and went off after a few minutes driving, the next day they never went off at all the following day they didn't come on. yesterday the car went to be diagnosed again and the fault code said it was the water coolant but this was ok. when you apply footbrake lights go off.