Hello, I have a 1997 bmw M3 and I have a few problems with it. The first problem is that the heat and ac unit keep coming on and off whenever. Sometime I try to turn it on but it won't come on. I check the wires it seem like all the wires are in good contact with the unit. I don't know what causing it. The second problem I have is that I my O2 sensor might be bad. Can I replace it myself and where does it located. Would you recomment do myself or have a shop replace it?