[John Rogers]

Hummmmm, upstream left or right or downstream left or right O2 sensors? Since ********.com shows different part numbers for most of them, they might give different voltages. I would ask the mechanic to plug in his tester and see that each injector, coil pack (or what ever they have) and O2 sensor is reading. Do this while driving to get accurate real life readings, not idle. When I was putzing with a failing cat I plugged in my AutoEnginuity software and watched all those items, one injector was hanging open it seemed and caused the cat to overload from the rich exhaust.