90 Cut Ciers: code 45 and stalling problem in reverse gear

I have a 1990 cutlass ciera, which still works well. However, Last month, the light of Engine service soon was on for a few seconds frequently. I asked people to help me check it and the instrument showed code 45: oxygen sensor rich. And I also found the oil usage seemed much faster than before, but not sure if it is connected with this error code. Any suggestions for this?

In addition, this car stalls frequently when it is in reverse gear state. This doesn't affect my normal driving in drive gear, but I am afraid that this may reveal some potential serious problems in it.

Besides, there seems a little trick thing in the Air condition. Every time I turn the AC on or off (seldom do this), something weird happen: a little shaking, large noise, or even stall one time. Again I am not sure which factor may cause this problem.

Any suggestions or comments will be appreciated.