The code p0141 shouldn't have an effect on fuel economy. That code just indicates a problem with the heater circuit for that sensor. You can resistance test the heater circuit in the sensor but considering you have changed twice it's not likely you still have a bad sensor. Sometimes when the sensor fails it shorts the heater to ground and will blow the fuse for it. Then after replacing the sensor there is still no power for the heater and it will continue to set the trouble code. There should be a 20 amp fuse in the underhood fusebox for the rear O2 sensor heater.