I have made extensive research on this one from the forum. Code 28 is the "real main O2 sensor". What made me confuzed is:
Half of the reply said it's real one of the two o2 sensors on the manifold, the other half of the reply said it's the o2 sensor on the cat.

Some serious help really needed!
Thanks

 

I just looked at my lexus tech info and it has a hint for D.T.C. 21/28. "if diag . code 21 is output, replace the front side main oxygen sensor. If diag. code 28 is output, replace the rear side main oxygen sensor." Hope this helps.