I've been getting codes like P1131 and P0171 that would normally clear out after cleaning things like the MAF. But now at over 100K the P1131 keeps coming back. The normal life for an Oxygen Sensor is around 30 ~ 50K. They can last as long as 100K, but they don't work very well at that point. Like on my car I think it is flat lining at random times. When this happens it puts the car into a mode that adjusts the fuel ratio to awaken the sensor. You really can't feel this change unless you are one with the car force Gods. Where as you may have gotten 22MPG in the past you would get something like 19 with the one Sensor going bad and with 2 bad things would get much worse because it can't read any data. Odds are you may drive years with a bad sensor, but nothing is stored until it's about dead. With that being said if your car is over 75K miles and those sensors have never been changed it might be a good idea to replace them before they kill the catalytic converters.
This is a good place to order these sensors.
http://oxygensensors.com/ 
