[a.graham52]

personally i would check the ohms of the front two sensors first. just because its easier to get to them and probably where you will find your issue. i work at a chevy dealer here and i know once in a while customers will come in and ask to have their codes checked by a service writer just to make sure there cars going to be safe and see how long before they have to come in for repairs. you could try and and see if you could get your codes checked for free..