A few weeks ago my car failed the emissions test, and was running terrible. I replaced the O2's and that was the problem, but now when I let the car idle long enough, like a red light, the SES light will come on, but only when it idles. It doesn't change the way the car drives, but could the new O2's have something to do with it? If so, how?

 

Its possible but until you get the car scanned all we can do is guess and thats not going to help.
Get the car scanned and then let us know exactly what code came up then we can help.