O2 sensors front rear, diffrent?

[mn box s]

I just replaced my exhaust full system with fabspeed test pipes. I was getting cel for 2nd bank o2 and maf so I replaced the MAF and ordered 4 new o2s I have headers now and the front o2 wouldnt reach so I used the rears and they pluged in. Fabspeed claims no cel. I have not had time to scan again and wondering if people have the same problem. Also hunting for idel badly 90% of the time. I now have only 2 o2s hooked up? CEL went off after install but came on with in a minute again.