P0421 intermittent - already have a spark plug non fouler in

[smetzger]

So, I am getting an intermittent P0421. Sometimes it will be confirmed and the CEL will come on. Sometimes I'll clear the codes and it will go through the whole drive cycle ok, but then 500 miles later I'll get a CEL.

I already have the spark plug non-fouler extension on my lower/2nd O2 sensor.

Would putting a 2nd extension in help?

 

[MrGiggles]

Adding another can help, but your O2 may be going out.

Another set of non-foulers is pretty cheap, I'd try that first.