I drive a 2001 Vauxhall Corsa 1

I drive a 2001 vauxhall corsa 1.2
I noticed my engine light come on. Car was driving perfect, I took it to Kwik Fit who diagnosed the problem as exhaust sensor bank 1 not working. They fixed the problem and I went on my way. Since bringing it too them my car idol speed on rev counter is lower then normal and has a few times dropped after turning on my car cutting it out completely, it done this several times before I managed to get the car running normal. The engine light came on another two times after this and upon bringing it back was told its the same problem and they apparently fixed it but now my light is on AGAIN. My petrol gage also goes from half to full to empty on its own accord and ive got a funny feeling that since bringing my car to them the first time they have messed in something else causing these other problems?