hi there we have a 2002 bugeye with 36,000mls. the cel light is on and code reads p0420 catalitic converter,system efficency below threshold,bank 1. as we are aware this is the code for a decat,ours still has all cats in,with only a cat back magnex system.we know its either a failed cat or 02 sensors,the car runs perfectly fine,any help would be great as we dont want to replace everything just to find the problem, or has any1 else had this happen while the cats are still fitted? we have had the light reset twice and comes back on after a week or so, and if we were to replace the sensors (which wont be cheap) would the light go straight out if they were the problem or would it need to be reset and wait see if the come back on, thanks

 

i have the same problem after fitting a ninja sports cat,
fault codes for o2 sensors are, p0031,p0032,p0037,p0038,p1030,p0131,p0132,p0133,p0 135,p0136,p0139,p0141,
i would get ur emmisions checked by a garage, i got my wrx checked by a local friendly garage and everything was fine. i was then told the car would be perfectly safe to drive and the only way to get rid of the light was to get it cancled when getting a remap. i bought a SCANGUAGE2 digital gauge which turns the anoying cel light off

 

I will take it down to my uncles and get the emissions tested asap, ur right though it is very annoying. also just had a full bottom end rebuild and somone said that this could've caused it to go due to incorrect fueling, as we had the light on when it went, is this true!!!

 

well had emissions checked and flew through passed lamba test and cat test,so now we are really confused. we have purchased a diagnostic machine of ebay so we can keep a check on the codes. and will keep a look out for some o2 sensors as 1 has to be slighty faulty.

 

glad to here all is well with the scoob
i would definatly look into a scanguage 2, it will give u all the parameters when the fauld occurs ie engine temp,throttle possition,engine load,boost pressure and many more plus it cancles the engine check light not bad for £125.

 

changed the rear 02 sensor today and the old 1 had a slight cut in the wire,so we are hoping this is what was throwing the cel on,fingerscrossed if not then will try the front 1.

 

I have posted elsewhere but P0420 has nothing to do with the front lambda sensor

Shaun

 

still no cel, so far so good