Code P0420

What does this code mean? Do I need a new cat, or just o2 sensor? Help plz!
2003 Toyota Tacoma 3.4L v6 164,052 miles
I going to add that I had just filled up on gas before the light came on?

 

Do a search on this website for po420 and read some of the threads that others have posted. Plenty of info out there. It could be bad o2 sensors or bad cats.

 

Try clearing the code and see if it comes back.

 

So I broke out a scan tool and I’m reading it all so here’s some of the data while Idling (700rpm Avg) at operating temp.
O2 sensor output (b1-s1) - 3.270V to 3.291V
O2 sensor output (b1-s2) - .095V to .740V
Short term fuel trim bank 1- 0.8% to -0.8%
Long term fuel trim bank 1 - 3.9%
Calc. Load value- 15.7%
Air flow rate from MAF- 3.69 g/s to 3.73 g/s
Short term fuel trim (b1-s2) 99.2%
Equivalence ratio (b1-s1) .998

My scan tool lets me test o2 sensors
Here’s what it looked like
(It said I couldn’t select the Bank1-sensor1 O2 sensor. It said the vehicle wasn’t compliant)

 

The cat(s) are baby sat by the rear O2 sensor. When you get that inefficient code it means the sensor dips below 700mv down to 100mv. If it fluctuates that low often enough it will trigger the code. From what your seeing looks like that’s not a constant problem as your staying on the higher end (950-740mv). With that you could probably kick the can down the road and just clear the code for now. Not the case with my truck unfortunately.

This is a REALLY good video on the subject, and you’ll also learn that replacing the sensor is a waste of time and money.
https://m.youtube.com/watch?v=vKLFO8MF7SY

 

P0420 means that the catalytic system is operating below efficiency but it does not necessarily mean the catalytic is bad. Catalytic converters don't go bad, they get destroyed by rich or lean running issues. So if you didn't have any of these, your catalytic converter is most probably fine

A very common cause of P0420 is exhaust leak. It can also be something as simple as a dirty or bad o2 sensor. Or worn out sparkplugs....

start troubleshooting with the simpler things first

 

I've seen you mention these types of comments on p0420 posts and respectfully disagree. Watch the video and read the comments/debates as to what a p0420 is really about. Bottom line all the little items you suggest would cause other codes. I dont disagree that they are precursors/causes to destroying a cat, but they do not generate the p0420 in of themselves. For example worn out spark plugs would cause a rich condition code P017X or misfire codes. Left untreated for long enough and the cat will be destroyed, the PCM will see the rear O2 sensor is mirroring the fuel trim variations and set the P0420. The PCM sets minor fuel trim variations so it can watch the rear O2 monitor for the presence of oxygen. Simply replacing the worn out spark plugs is too late at this point. Once a P0420 appears, the cat damage is done. Often times the OP will either not mention a recent repair, not be aware of their poor fuel trims, or have just bought the vehicle not knowing the history and simply sometime later inherit the inefficient cat.

more examples:
Bad O2 sensor? you get a different code for that. And it would not cause the rear O2 sensor to mirror the fuel trim variations.
Dirty MAF? you get a different/lean code for that. And it would not cause the rear O2 sensor to mirror the fuel trim variations.
Exhaust leak? you get a different/lean code for that. And it would not cause the rear O2 sensor to mirror the fuel trim variations.

 

Clear the code first. I got this code over a year ago, cleared it and it hasn't come back, passed smog too. It could be an anomalous event.

 

I'm no expert with P0420 codes, so I might be wrong on some points, I agree, but my experience with p0420 was caused by exhaust leak. The O2 sensor readings were fouled, hence why it was trying to tell me the cat was bad when it wasn't. Pipe was cracked right behind the cat. Cut and put a new flange and pipe back and code didn't come back since

but anyway, I was just trying to help. Thank you for being respectful though. I appreciate it. Some people just bash on others when they disagree or when they want to tell them they are wrong. Thank you for not being like that

 

simple to explain... the pre cat O2 sensor fluctuates quickly the post cat O2 sensor should have a slow steady reading. If the post cat O2 sensor has sharp changes the cat isnt working as it should. Clear the code and see if it reappears.

 

My recent experience with P0420:
I have cleared a P0420 code approx 8 times in the last 2 months. It has come on anywhere between 5-20 miles since the last time I cleared the code.

Easiest things first, to check for exhaust leaks, I ran half a can of seafoam through the intake via the brake booster vacuum line and followed the white smoke. I found the muffler itself leaking badly and the rear O2 mount leaking badly. I ordered a Walker intermediate pipe and muffler/tailpipe combo through Summit and installed them yesterday.

I drove 25 miles to work this morning and no code popped up. This is the only time a code has not popped up in the last 2 months with driving over 20 miles(it sits in my garage 95% of the time).

Now, I don’t consider it fixed until I put a lot more miles on, but I am encouraged by my drive this morning. I’ll report back to this thread in a few more weeks after putting legit miles on it.

 

Update to above post: drove 125 miles today and no codes popped. It looks like a P0420 code can be triggered by just exhaust leaks as some folks have mentioned. As optimistic as I see this, I’ll feel more comfortable after 500 more miles.

As a side note props to Walker Exhaust for making a very reasonably priced intermediate pipe + muffler/tailpipe combo. Less than $175 in parts and all fit good. I don’t know how long it will last, but it should be a few years at least.

 

my Walkers last about 4-6 years depending on conditions. I'm on my 2nd one and the rear hanger just rusted off, so bought a clamp top hold it

 

I had a recurring p0420 code last year that would come on randomly after filling up sometimes, I would clear it and eventually after a few hundred miles or more it would come back.

About 6 months later it would come on and come right back after I cleared it, eventually found out I had a bad front cat and replaced both which got rid of the code once and for all. That was over a year ago now actually.

It will keep coming back if you don't take care of whatever is triggering it, that much I can assure you. It's almost always either a bad cat or leaky exhaust. O2 sensors throw other codes like p0136/p0135

 

I have been driving around with a P0420 code and the "cylinder 3 misfire" code for about a month in my '02 Xtracab. I inspected for cracks or leaks but couldn't find any (not that it was the most thorough of inspections). I finally just purchased a new bolt-on bank 1 cat (from Eastern Catalytics based in North Carolina), and new spark plugs and spark plug wires as well to hopefully get rid of the cylinder 3 misfire code. Replacing cat tonight. If that doesn't work I'll have to go back and research more threads like this on TW. After installation I'll clear the code and see what happens. Glad I'm not the only one experiencing this code, seems fairly common. The video above was super informative. Will update post.

 

Do you really have a misfire or is that some old code? Is the truck running ok? I wouldn't replace the cat until the misfire is fixed as it could damage the new cat.

I have the $10 OBD Fusion iphone app which has all the live sensor data for OBD2. When I had andriod I used Torque. I also purchased a $10 in-app purchase to get the Toyota specific metrics which include nifty things such as misfire counters per cylinder, transmission fluid temperature, airbag codes, etc. Having a scan tool that shows misfire counters will help you validate how frequent the problem is and whether you have fixed it. With basic scan tools that only read codes you may have to wait longer to see the code pop and diagnosis time comes to a screeching halt. Also dont forget the long screwdriver to the ear test to make sure the injector is clicking and sounding like the others.

 

I wish I would have known sooner to rectify the misfire code before i tackle the cat issue. A little too late in the game I guess since I just replaced my bank 1 cat and the spark plugs and spark plug wires simultaneously. I know it seemed sudden to jump the gun and replace the entire cat, but a lot of members were reporting no change after replacing the O2 sensors and still unable to clear the cat issue. And the guy in the video above mentioned at the end of the video that a P0420 code usually is solved when replacing the cat. I'm pretty positive the previous owner neglected some regular preventive maintenance, and now I'm playing catch-up. We'll see if the code clears. Thank you for the information about the scan tool and Toyota specific metrics. Interested in hearing more about OP's truck since his alterations, too.

 

I just bought a 2000 and had 401 & 420. I was in a bad mood but began to tackle it ...reading TW. First thing I did was fill it up with high grade gas, I think it was 93 octane. But reading more & more, I wanted to do a visual inspection, got under the truck and began looking around before throwing parts at it. There was a major leak between cat2 and the muffler. I put a gasket in there and sealed it with some silicone (not too heavy) and also my O2 (downstream) had literally no gasket whatsoever. So I got a new one and redid it with no-seize thread compound. It fixed the problem. I have not seen 420 ever since. I also tackled the 401...that proved to be a bit more challenging.

 

Yes I’m the one who posted the video and agree that’s there’s not much escaping it being a bad cat, so you likely fixed the p0420. But if the misfire is not fixed then the engine may keep dumping fuel into the new cat causing a premature death. How’s the trucks sounding/running with new plugs/wires?

 

Wanted to wait a few days before I responded with an update so I could drive it some. Sounds great now. When I replaced the cat, the P0420 code went away but then immediately the code for bad air/fuel sensor lit up. So I replaced the sensor and figured I should replace the downstream sensor too while I'm at it. Now I've driven 100 miles, no sign of CEL returning. Wish I would have just replaced the sensors to start off with and possibly could have saved myself $400, but you live and learn.
OP, how's it going on your end with your '03?