I have just installed LG Pro long tube headers on my 03 Z06. I used HP Tuners to turn off the MIL errors (P0140 and P0160) and SES light for these codes since I do not have cats nor rear O2 sensors. I was looking in a product catalog last night (Mid-America) and it said you need to use O2 simms in the rear when you don't have cats - else "your air/fuel ratio will be processed incorrectly". I was under the impression that the rear O2 sensors were only used for emissions purposes to determine if the cats are functioning properly. Which is correct?