Just got a CEL & O2 sensor code!

How long should I expect Denso O2 sensors to last?

I replaced mine 2/2011 @ 167K miles & now at 222K miles 9/2014 my CEL is on and the thrown code is for the O2 sensor(s).

I don't mind replacing them if necessary, but I could think of other things to do with $100. (Amazon price for both).

The original sensors lasted for 165k miles (10yrs)! So, I was expecting a similar life out of the ones I replaced ONLY 3+ yrs ago.

 

Sometimes its not always the 02 sensor that is bad. There is other problems that can cause it to throw that code. What is the code?

 

P0420
I'll clean my MAF tonight and maybe that will solve the CEL, I will also change the spark plugs over the weekend.