remauto1187
Shooter
The pre converter o2 sensors are the ones that are used to measure the oxygen level and feed a signal to the computer so it can make air to fuel adjustments by changing the pulse width (Open/Close time) of the fuel injectors. The post convertor 02 sensors just measure the oxygen level and that value is compared to pre convertor values via the computer and this info is used to alert to a degraded convertor performance. The post convertor 02 sensors will throw a "poor performance convertor" code/light but they have nothing to do with the engine running good or bad and DO NOT affect gas mileage.