P0141 "Rear 02 Sensor"

This code has popped up twice now. I erased it the first time only for it to trip back on two days later. I don't think it's the wiring to the post Cat 02 sensor because wouldn't the trouble code immediately come back? Interestingly, I just installed a 180 degree Thermostat about a week before this started, not sure if that could be related. From what I have gathered, this code means that there is a built-in heater inside the sensor that is failing or has failed. I'm also assuming that this sensor has been never changed since new in 1997 so how hard will it be to unscrew this thing out? Any special tools that I need? Is this sensor only there to monitor the cat? Does this particular sensor affect gas mileage? Of course, If I need to replace it, I'll go with OEM, not Bosch.
Thanks in advance for any suggestions and tips...

Paul, that sensor is located in the infamous exhaust u-bend, and is a bear to get to because it screws in straight down on the top of the pipe. From what you describe, it sounds like the sensor's heating element has failed, but it wouldn't hurt to inspect the wiring first. The post converter sensor doesn't really effect performance/mileage unless it's shorted out. You know where to get an OEM sensor when you're ready to change it.