P0420 MIL

[viper1230]

Last Sunday I got a P0420 code. I got home and scanned the code and thought it meant the O2 sensor bank 1 was bad. So I bought a sensor (not yet installed) and cleared the code. Since then the code hasn’t came back on and car runs perfectly. I just got done looking the code up and watching Eric’s video about catalytic converter diagnosis, however, I don’t think that it is my cat. I say this because just a year ago I had a whole new exhaust (performance) put on that included a new cat and downstream O2 sensor. There are no exhaust leaks that I know of. However if there was it would be post cat as everything before then is welded, minus bolting the manifold to the block. Should I change the upstream O2 and see what happens?

Topic

[ridinred24]

your downstream 02 is the one that monitors the cat. The upstream adjusts the fuel mixture. Chances are pretty good that a crappy aftermarket cat was installed. Check the inlet and outlet temps as shown in erics videos and report back

[viper1230]

I personally bought the whole exhaust system and everything but the header is magnaflow. But I will have to go get the temp thing(can’t remember the name right off hand). I just hope it isn’t the cat lol!

[R.G.Young,Jr]

O2 sesnor bank one has to do with fuel control. if you have a scan tool that show live data, look at the short term fuel trim along with that particular O2sensor. the lower the O2 numbers the higher the short term fuel trim numbers will be.
If your meter want show live data, another test you can use to see if the O2 sensor is working and working properly, take a dvom ( digital volt ohm meter), turn the scale on dc voltage down to the lowest scale that will show at least one volt. take the black lead and attach to a good ground, then take the red lead and back probe the Bank one O2 Sensor connnector signal wire. once your connections are completed, start your engine, and observe the readout. your readings once the engine is warm should be switching from .1-1. this is mila volts. what you are reading is 100 mila volts to 1 full volt. as you observe the screen on the meter, and if it is switching, besure it is going above and below .5 mila volts. if it is not changing, then create a vacumm leak and see if this will cause it to change to below .5mv. if it won’t change close the vacumm leak,that means the sensor cannot recognize a lean condition,next you can take a propane bottle and attach a small tube to it and introduce a small amount of propane into the intake and that should cause the reading to go high. if the sensor fail either or both of these test, then you should replace the sensor.
Be sure to look up the wiring diagram to besure you are on the O2 signal wire,there may be more wires in the O2 connector, they would be for and O2 heater, be sure not attach to these for this paticular test.
Also be extremely careful useing the propane.
Take your time and be sure you have everything in place before you start your engine, you should be able to get the answers you need in just a few min. after you start the testing process. let the evidence dictate what you need to do.
Keep us posted as to what is happening, and good luck.

[college man]

P0420-catylist efficiency below thresh hold.
Take the o2 back and get your money back. Two
things. #1 The exhaust that you bought is a high
flow exhaust/cat which increases flow to the point
that the cats o2 sensor can’t detect flow.so it sets
p0420. #2 cat has lost its efficiency. what vehicle
are we working on?

[viper1230]

It is a 1999 Pontiac sunfire gt with the 2.4 DOHC. Don’t know if it matters but it has 116k miles. I do not have a bidirectional scan tool but I have a dvom so I will do the probing and see what it gives me.

[Jim]

college man is right. The aftermarket cat has to much flow. This code has nothing to do with the position 1 o2. GM vehicles don’t like aftermarket cats. i would see if you have a warranty on that exhaust and get it changed but beware that the code will probably pop up again in the future.

[viper1230]

So if it is just throwing the code because the O2 sensor doesn’t like the high flow of the exhaust then I could just keep clearing the code and forget about it? I do not have a warranty for the exhaust nor do I have the $ to change the cat.

[Jim]

ya u can clear it but be aware that it will come back

[viper1230]

Ya I kinda figured it might come back, but I’ll just Clear it again lol

[Author]

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