P0420 Catalyst System Efficiency Below Threshold
 

hi need help basically driving along and the engine light came on but car was still running ok, hooked it up to a fault reader got the following code <P0420 Catalyst System Efficiency Below Threshold >once the code was reset the car stalled after it was started a few times plus the idle wasnt brill plus alot of pops and bangs when the car is reved slightly so basically over fuelling, and ideas ? put a multimeter on the front lamda sensor where it plugs in and gettin a stable 3.8v does that mean it knackered ? car has a full decat 3" system and a blitz dump valve dunno if this would cause this , its an 2002 sti, cheers in advance for the help

Hi, A cat efficiency code will generally tell you your cat is fupared. But as say you are running a decat system. How long have you been de catted. ? Have you been mapped?? Can you tell us a bit more about your car, what mods etc?

That code will keep popping up as you havent got a cat.

A remap will be able to switch it off permenantly (and get a lot more power/torque for you)

its been running a full decat for at least 4 years and a full 3" for the past 2, the car was remapped thro re motorosport in leeds, so basically a dont think its because a dont have a decat, guy before me put a RCM ceramic coated downpipe and wraped headers plus got it remapped to 330bhp said the guy that did the remap was the scoobyclinic mapper thats where the rolling road graph came from so ad think they would have tweaked in what you said , basically thinking its 1 of my lamda sensors

If it had been remapped with the decat on the mapper would/should have switched the CEL light off for it.

It is coming on because there is no cat. So your map needs changing to suit. It will keep coming on because of that - no other reason.

why has it ran fine for 2 years ?surely it must have been mapped in or it would have come on by now , plus it wants to stall when first started and its over fuelling can smell it and here it pop when u rev it

The front lambda sensor would throw a different fault code to P0420 from your symptoms it sounds like that is faulty.

should the voltage on the front lamda sensor be stable it 3.8 or should it go up and down ?