Winter blend. Your engine may not be running in its most efficient mode either. Tune up? Timing belt? Fuel fitler? valve clearance? upstream/downstream sensors? etc etc
If it's on or after the catalytic converter it's considered the "downstream" sensor. This sensor measures the efficiency of the converter.
If you have a non-Calif model, then the Bosch planar sensor (universal type, #15733) at $57.79+shipping from rockauto.com is a very good deal. For $10 more you can get one with OE connector so you don't have to splice the old connector on the universal sensor. The planar design also resists contamination much more than the traditional thimble type. Autozone has free loaner sensor socket tools. Stay away from the Denso. IMO they're junk.
The check engine light was on for 4 months, hadd it checked ans was
> determined that it it the one on or near the cat converter. Could
> not afford to replace. Spring has returned and the light is now out. > What gives?