P0420 Catalyst System Efficiency Below Threshold


I needto know what this means? I had to install a new O2 sensor today and my check engine light came on. I also installed it to the old wiring harness I may have conneted the wires on the wrong wires. The part is a bosch O2 sensor could not get an OEM from the parts store.

 

That means you have a bad cat. Now since you said you just installed an O2 sensor, it might be that. Which O2 sensor was it that you replaced? What kind of O2 was it? Did you have to cut the connector and splice it into the new O2? If so than I think that is it. If it came with a connector already and you didn't have to cut any wires than I think it might be the wrong O2. Now depending on which sensor it was, you might be causing harm to your engine. If it was the one after the cat, all you are doing is reporting that the cat is bad. If it was the one before the cat, you are making your engine run unsafe mixtures and it could be harming the engine. Go over the installation again.

 

THis is alomost a daily code im my mitsu. Are you running the stock cat still?

 

The part is a bosch O2 sensor could not get an OEM from the parts store.

No i have an OBXR High flow cat.

That means you have a bad cat. Now since you said you just installed an O2 sensor, it might be that. Which O2 sensor was it that you replaced? What kind of O2 was it? Did you have to cut the connector and splice it into the new O2?

I just replaced the cat and the old o2 sensor was to short and i left it hanging, and a couple of days ago it started to bounch off the ground and bet up the underside. I cut the old wires and spliced into them and about 10 to 20 miles the light came on.

Would it be also that i have a DC Header the bung for the O2 is just before the flange and the cats is after O2 Spot is just after the flange?

 

Mine goes off because of the HFC.


I would have to guess its your O2 though


How long has the cat been in there?

 

its been in there for about 3 weeks

 

I am still a little confused as to what you did exactly. You never explained which O2 you replaced, and now you mentioned you left it hanging. Well, first let me say the O2 sensors are sensitive components. You shouldn't leave them hanging, or disconnected while driving the car. That is a sure way to damage them. So first of all, I will say, even if the sensor is only 3 weeks old, if you left it out of the exhaust and let it drag on the floor, or if you left it in the exhaust and unplugged and drove the car around that sensor is now no good. The location of the O2 sensors is very important. The first O2 sensor has to be somewhere before the Cat, and never ever have a bung directly on the bottom(at the 6 o'clock location). Water can build up(and drop down to that location) and damage the sensor. The second O2 sensor should be somewhere after the Cat but before the muffler, and also never on the 6 o'clock location. Now to troubleshoot your problem you have to understand what each component does. The first O2 sensor controls your air fuel mixture. If you are starting to get bad gas mileage, then I would syspect that sensor is bad. If you still get the same gas mileage, same horse power, and no weird exhaust smell, then I would say either the Cat is bad(but since it is new I don't think so). Or the second sensor is bad. Double check all your connections. The best connection is soldered with heat shrink tube. If that is not an option then make sure you get those good crimp connectors. Good luck.

 

The O2 sensor is the one that is suppose to be after the cat that i replaced. The thing is who ever made the cat they ****ed up and I have to take it to a muffler shop seeing that my 2nd O2 sensor is reading what the first one is. I might just make a O2 sim. for 2nd Sensor.