CEL, P0420, Catalytst Efficiency Below Threshold

TomZ · Post by **TomZ** » Wed Jan 10, 2007 8:40 pm

G/F calls me at work to say her CEL is on. I have her go by the local Advance Auto Parts where they report DTC Code P0420, Catalytst Efficiency Below Threshold. The counter gal says it's an O2 sensor. Looking at the FSM Emissions Section for the 2000, it seems to me the cat would cause this code, or a bad sensor. The FSM references a dual O2 system. My '02 only has a single sensor, I've never looked to see if hers has a dual system or not. The '00 is a high mileage car, so either scenario is possible. Anyone have any experience with this? Since my O2 sensor was $90, I'd like to only change the bad part, lol.

04sxt2.0 · Post by **04sxt2.0** » Wed Jan 10, 2007 10:04 pm

i've seen this code alot, but only on nissans, normally o2 sensors throws a different code, could be wrong, but how many minles on the car, original cat? most of times i see that code it is related to the cat. your best bet would probely be to go to the dealer and have them run the code normally they have TSBs posted on codes, and more than likely they can tell you exactly what is wrong and how it can be fixed

MoxHair · Post by **MoxHair** » Thu Jan 11, 2007 7:45 am

I have this code.. But thats because I dont have a Cat.

00element10 · Post by **00element10** » Thu Jan 11, 2007 8:21 am

yeah the 2000's have two sensors, one in the exhaust manifold and then one in the cat.

and unless you took the cat out of your car you should have a second o2 as well no?

quicksilvr · Post by **quicksilvr** » Thu Jan 11, 2007 6:33 pm

00element10 wrote:yeah the 2000's have two sensors, one in the exhaust manifold and then one in the cat.

Uh, ALL our cars have two....an upstream in the manifold, and a downstream in the cat. TomZ, if you only have one, it better be the upstream one, cause the car won't run very good without it. I've tryed.

ALthough, you can definately get away without having a downstream sensor...I haven't had one since I installed my long tube header.

SILVER-ES · Post by **SILVER-ES** » Thu Jan 11, 2007 6:46 pm

When I had my neon I got this code alot due to my downstream o2 sensor being before the cat with my OBX header installed. It was a pain in the ass code because it would put the car into safe mode. I'd start with that sensor and go from there.

silentneon01 · Post by **silentneon01** » Thu Jan 11, 2007 6:56 pm

got this code a lot changed the senors 136.00us cause the damn thing is shorter then ones they sell at Autozone, advance and such. Didn't fix it. Due to miles at the time 90k+ had my guy check it out when I had the timing belt done saw poor flow thru cat. changed out cat code went away.

So its your lower senor or cat or both. Fastest fix may to be to pull senor and spray it with carb clear but not much and see if that helps. Took care of my code for another 3 months before I actually fixed the problem.

03blackrt · Post by **03blackrt** » Thu Jan 11, 2007 10:52 pm

MoxHairR/T wrote:I have this code.. But thats because I dont have a Cat.

gilly02le · Post by **gilly02le** » Fri Jan 12, 2007 12:09 am

had this same problem, you'll need to replace the downstream o2 sensor, (the one in the cat).

ragek23 · Post by **ragek23** » Tue Jan 16, 2007 2:07 am

i am having the problem with the obx header in my 02. I had the obx installed with the aftermarket downpipe. It worked great but with no cat i couldn't hear myself think. I installed the stock downpipe to quiet things down and I think it made my car go into safe mode as mentioned above. My only option was to disconnect both the downflow and 02 sensor. Is there any way to fix this besides running the stock headers?