it was and then i cleared it and it hasnt come back on

 

im not running cat or muffler on mine and love it. No lights on 07 5.9 CTD

 

ok what are non foulers am new to this my mustang had no cats and I did not get a light problem so I did not worry about it.

 

Always check out and read our FAQs. Chances are, everything is already answered for you.

HERE is a link to the FAQs

How to eliminate CELs when removing CATS...

 

Yep like they all said, i bought a Magnaflow high flow cat and had the same problems as what you are havin on your 5.9....light would come on and throw a code of catalyst efficiency below threshold on bank one..so i put in the non foulers and never had a code pop up anymore..it is def the fix if you are runnin that mag high flow cat...dont think that your cat is bad b/c its not..just the way it is...good luck

 

did yours go bad or is it just the code because my truck drives fine

 

nope mine never went bad at all..its just b/c of the high flow that it is not knowing what to do with all of it and so once i put in those non foulers, i never had a problem ever again..loved my magnaflow cat

 

A rear O2 CEL does nothing to hurt driveability unlike a pre-cat O2 will. It is simply an annoying light that tells you that your Cat is operating out of a pre-set range.
The only bad thing about leaving a post-cat O2 CEL alone is that if you develop another unrelated issue, it could very well go un-noticed...

 

yup thats what i was thinking

 

Leave it on and do the non-fouler trick mentioned above and you should be fine.