First of all you did a great job of laying out the information in regards to your concern. My first assumption would be that your vehicle has not been properly diagnosed. Commonly I see people scan their vehicle by a facility (or parts store that offers free diagnostic code readings) that do not specialize in diagnosis. They will pull a fault code (likely P0421) and say, wala! It needs an oxygen sensor. The issue with this is that it is not the proper way to diagnose an issue. Your engine computer says it has a bad reading from the O2 sensor, however, how do we know if this is caused by the sensor itself? Yes, most commonly it is the sensor, but without proper component testing, you will continue to have an unresolved issue. First step is to have the vehicle properly diagnosed. If you can not afford diagnosis at this time, then return all your O2 Sensors and use that money to pay for diagnosis. Another option (provided you have access to the equipment) would be to take the front and rear temperature of the catalyst to make sure it is heating up. Then scope the O2 sensor to check for irregular patterns. Finally, double check to confirm that the O2 sensor wires did not get swapped when replacing (a common rookie mistake)
