I'm having problems with my trouble code P0420?

Question

Both of the O2 sensors are fairly new. I've changed them in the past year. I reset the code after I changed them, and the code came back on.

I went and got a smog test today and my van failed because of the light being on. But, it passed the emissions test, and the mechanic said that if it was the catalytic converter, it wouldnt have passed the test. If it's not the O2 sensors, and it's not the catalytic converter, what else could it be?

I read online that I should check the voltage of the O2 sensors. How do I do that?

Help me out, please.

Answer 1

The 02 sensors must be working correctly so the problem may be upstream as the sensor only reads whats in the pipe. so you may have another problem, but it affects the 02 sensor first. You may want to try cleaning the MAF sensor with the correct material so you do not damage. But is hard to pinpoint without hooking up to a scanner to "read" what your engine is doing. Also would be helpful to know year.make and mileage. But best to take to a mechanic you trust and have hook up to a scanner to fruther analyize the problem. Note: a scanner is not a simple code reader

Answer 2

The catalytic converter is bad on bank 1 which is side where the cylinder number 1 located. if you have a v6 engine with two converters you can connect the bank 1 with Bank 2 by cutting the black wire on the 02 sensor on bank 1and and leave the 02 sensor disconnected and connect the black wire that goes to the o2 connector to the black wire on bank 2. So what will happen is that it will fool the ECM thinking the both converters are working but if that bank 2 goes bad it will throw a trouble code for both converters are bad. the ones you want to do this with is the sensors that are after the converter.. if thats not an option you can get an o2 simulator for $25 but if you have to pass an Emissions Test you have to get your converters replaced if one is bad the other is not far behind. I was able to do this trick on a 1997 ford taurus and still no Check Engine Light.

Answer 3

Ok first take it to autozone and have the code reset and then drive it again and if the light comes on have it scaned again.
One thing here you my not know.
If you buy aftermarket o2 sensers, in some cases thy will not work in some cars.
For example an aftermarket o2 sensers for a toyota will not work.
The sensers you have gotton may not have been good in the first place here is why

If you got them from Autozone here is how thy may be bad.
Lets say that you put in new ones but lets say thy sensers were bad because the person that got them befor you put in the new ones and that persons put his bad ones back in the box and took them back to the store. In most cases thy will not open the box to see if thy are the old ones or the new ones.
Beleave me being a mechanic I have gone to auto zone and got a part, open the box in the store and found that it was a used part
If you got the new part took it out of the store, installed it and it was someones use ones it will still give you the bad code.

Hope this helps and answers your question.
.

Answer 4

you need to measure the resistance thru the o2 sensors, u can use a dvom meter or (digital ohms volts)