P0420 troubleshoot

I've gone through many of the other posts trying to diagnose this code. I'm fairly new to this site/working on my own truck but very eager to learn.

So here is the rundown. The truck(2000 Prerunner) has 150k miles on it the previous owner blew the head gasket (repaired long ago). I replaced all the fuel injectors with Bosh fuel injectors ~ 12k miles ago. The truck was and still is running fine. Out of nowhere, the CEL came on with P0420. I checked the forums and seems to me it could be a number of things. I called a mechanic that my buddy had recommended and he told me that it had to be the cat and said it's rarely anything else. After reading online some more and considering the current prices for cats in CA is ridiculous, I decided I want to check other things off.

I replaced the air filter. I checked for leaks in the exhaust system(visually) on a cold morning and found one right in front of the second cat weld. I'm hoping that is the only problem and once I fix it the problem goes away. do you guys think that leak could cause that code? I also want to check for other leaks using the soap and water tactic but not 100% on the best way to do that. Also, I have a pretty cheap obd2 scanner that reads live data but not in a graph of the 02sensors. The front one when driving on the highway will fluctuate between 0.4 and 0.7 I read on one of the forums it should be between .1 and.9 it could be my cheap scanner not picking up the signals fast enough or something so I'm not sure what to think about that. The back sensor fluctuates a little but it's pretty steady at 3.29-3.3

My goal is to fix the leak tomorrow morning and hopefully find a better way to check for more leaks. then if that doesn't fix it maybe changing the sensors.

once I make these repairs should I erase the code myself or will the truck erase them automatically if the problem is solved.

Thanks for reading. Any help would be much appreciated.

 

Exhaust leaks upstream of sensors can cause P0420. This includes the manifold. Make sure you know if the sensor being tested is O2 or Air/Fuel and use appropriate test.
The emissions sticker under hood will list Federal or California emissions.
Run your specs to determine which sensors at link below

https://densoautoparts.com/find-my-part.aspx

 

I think you're mechanic is probably right. I had a P0420 that came and went for awhile, finally replaced my cats and got rid of it for good.

I got the 50 state legal Magnaflows

 

But do check for exhaust leaks before the O2 sensors like mentioned above, because that can throw the code as well

 

Yes, it can be other issues compounding a larger one. The cat is likely the issue, but it could be caused by something else, such as a cracked manifold. Both would need replacing in that case. Went through this with my last pickup.

If you can't accelerate past a certain RPM without a flashing check engine light, you've definitely got a clogged cat.

 

A burnt out cat missing material in it can be the cause as well, and you wouldn't notice any performance issues. But the rear o2 sensor will pick up levels it shouldn't be and send the code. That was my issue

 

correct me if I’m wrong the o2 sensor upstream of the first cat is the Air/fuel sensor and the the second one after the second cat is also an 02 sensor.

 

I have no acceleration issues or performance issues. At least none that I can tell.

 

The rear o2's only job is to make sure the cats are functioning as they should.

 

To my knowledge the rear sensor is operating fine based on the voltage.

 

They could be coming. Flashing check engine light usually means an active misfire which can be caused if the cat is plugged up enough. Another test you can do is go down and strike or try and shake the cat. If there's a rattling sound, you also need a new cat.

 

It will send the pO420 code though when it detects levels in the exhaust that shouldn't be there (getting through a bad cat for example)

 

The sensors front and rear get carbon coated and sometimes don’t work correctly.

I have 663,000 miles on my cat so check everything else 1st

 

I hope for your sake it's not a bad cat either, I know how expensive it is when it happened to me five years ago and I'm sure they're even more expensive now. But in my experience that's what it turned out to be

 

I did have a pO420 code that was related to my pcv valve, NOT CAT.
If I remember I had a bad hose connection so I replaced the pcv valve and gasket and hose and that fixed it

 

I actually bought a new cat and the code was still there coming back.

Returned the cat,

Replaced the PCV parts and good to go

 

I’m pretty sure I recently replaced the pcv when I did my fuel injectors but I’ll check that again.

 

Someone having trouble with 420?

 

Check the connectors on both ends of that hose and the gasket on the valve

 

Haha I wish it was that kind.