po420
 

could a bad fuel filter cause code p0420 catalytic efficiency below threshhold.has anyone dealt with this without changing the cat,i have a 2000 sir the light came on today,cleared it back in a few hours.

Could be a toasted primary or secondary O2 sensor. The stock cats last a long time. Should have a shop run a diagnostic with a proper scan tool to see if both O2 sensors are working correctly. If so, then it's the cat that needs replacing.

Not a bad idea to do the fuel filter if you haven't done it in a while, but it's unlikely it would cause that code to show up.

p0420
 

thanks had it checked out needs an o2 .not that bad thanks for your help.