My rear O2 sensors have been putting out low voltage for months which made the computer think that it was running lean, when in fact it was running extremely rich. I replaced both sensors and still had the same problem. I have aftermarket exhaust, headers, and CAI so I had my car dyno tuned. The tuner shut off the rear 2 sensors and I was finally getting decent air to fuel ratios. But a few days later I ended up getting the codes P0131 and P0151 (low voltage from my front 2 sensors). And I am still getting very poor MPG's. What would cause both sensors to fail at the same time?
Are the converters still attached? Check intake vacuum if at 15" or below it may clogged converters or lack of converters if not there. Also what is CAI? If you can check voltage with a scanner check for 300-600mva at idle if you don't have that then disconnect them and with key on only check for 100mva then check for a short to the sensors, then finally check fuel pressure for being low, vacuum leaks an exaust leak which is highly possible with headers and silicone contaminated O2 sensors which If the engine was apart and this may be the case with to much silicone sealer used. Fuel pressure should be 55-62psi also if you have one of the K& Ntype air filters, clean it and dont' oil it this time then use choke cleaner and clean the mafas it may be oil contaminated as well. Oh and when you modify these engines don't expect to get the same fuel mileage you did when stock.