Bound to happen. I finally got my first P0420 code!

I knew sooner or later I was going to be facing the P0420 code. Well yesterday evening, after I dropped off an employee, the Ultragauge beeped, and showed "Pending Trouble Code". I went into the menu, and there it was: P0420. So I will be starting this thread to go through my full diagnosis of this code. I read about so many people out there who have to deal with this code, and getting confused. Over the next couple of days, as I have the available time, I will go through and post all the steps, and findings during the diagnosis. Stay tuned!

 

Reserved


Common questions I receive:
What does P0420 mean?
Do I need to replace my cat?
How does the computer know my cat is bad or not?

P0420 has been one of the most undesired codes for any vehicle owner, be it Toyota, or any other manufacturer. They can be tricky for some people to diagnose and can be expensive to repair. Since 2006 all vehicles sold in the US are required to be OBDII compliant. This means that no matter WHAT communication type is uses (CanBUS, ISO, etc) certain trouble codes must have the same number, and same meaning. So a P0420 in a Toyota means the same thing as a P0420 in a Chevy, Ford, Hyundai, etc. It also means that the threshold minimum standards are all the same for each code. This means that if you can diagnose a P0420 in a Toyota, you can do it on any other vehicle.

What does P0420 mean? P0420 is defined as: Catalyst System Efficiency Below Threshold (Bank 1) EVERY vehicle will have this code available. Certain "V" configuration engines (V6, V8) will also have P0430 if there are dual cats NOT in sequence. The Tacoma with the 4.0 V6 has a cat for each "bank" or side of the engine. Diagnosing P0430 will essentially be the same.

The first step in diagnosing the code is understanding HOW the code sets, and how the ECU runs the test. The major components of the P0420 code are:

1. Three-way catalytic converter
2. The ECU
3. A/F sensor
4. Post-cat O2 sensor

Three way catalytic converter

It would take a good bit to explain the intimate details of how a catalytic converter works, so I will simplify it enough to this: It take the exhaust coming from the engine and chemically alters it to reduce harmful chemicals from entering the atmosphere. The worse condition the exhaust is, then the harder the catalytic converter has to work to "clean" the exhaust.



What does this mean? You should always make sure that your engine is running as efficiently as possible. This means proper maintenance. Change your spark plugs as recommended, and keep your air filter cleaned, or replaced. A dirty air filter can actually cause catalytic converter problems. An engine running at a perfect (stoichiometric) air/fuel ratio of 14.7:1 will produce the least amount of harmful gasses in the first place, so the first step in checking for a problem is to make sure your engine is running efficiently.






ECU

So how does the ECU figure out if the catalytic converter is doing it's job? First off, the ECU will never test a cat that is not warmed up. A cat cannot do it's job if it is not at temp. So trying to "test" for a P0420 without warming up the car will give you inaccurate results. After the vehicle is warmed up the ECU will also wait to run this code diagnostic until you are cruising above a speed between 40 and 70MPH. It will NOT however typically run the test if you are at a steady cruise speed. The ECU will also only perform the test ONCE for the drive cycle, so once it runs and either passes or fails, it won't test it again until the ignition is cycled. It take approximately 30 seconds to perform the test itself, but it may take a full 10 minutes of driving to run it.


A/F Sensor
The A/F sensor, while looking identical to an Oxygen sensor, works under a narrower and more "detailed" inspection of the airstream in the exhaust. It not only measures the oxygen content, but is also capable of minute detections of variations. Perhaps the simplistic version that would be easier to understand is imagine your TV is the ECU, An O2 sensor give the TV a 720P picture, and the A/F sensor gives your TV a 4K signal. This means that the ECU can make much faster, and more detailed adjustments to the fuel ratio, making the engine run more efficiently.


O2 Sensor
The O2 sensor after the catalytic converter measures the airstream AFTER the catalytic converter has "cleaned up" the exhaust. It does not need to be as detailed as the A/F sensor, because it's sole purpose is to monitor the catalytic converter. It has no effect on fuel trim, or anything else. It simply monitors the Catalytic converter.



Self diagnostic process

So while you are driving along the ECU will enrich, or lean out the fuel mixture. This alters the exhaust content that enters the exhaust system. The ECU will expect to see this change in the A/F sensor before the cat, and it will expect to see very little change in the O2 sensor after the cat. Anything out of what is expected (Based on the pre-programmed data for the code test) will flag a potential code. (This is when your Ultragauge beeps with a "pending code) This failure must occur twice, in two consecutive drive cycles, before it will set a hard code, and turn on the CEL.


All of this leads us down to the main question: How do I diagnose the code and replace the RIGHT defective part, and not just be replacing parts on a guess?


A poor condition A/F sensor can cause some chance of a P0420 code, realistically it is the least likely culprit. In my 25 years of being a tech, I have only ONCE replaced a sensor before the cat for a P0420/0430 code.



A poor condition O2 sensor after the cat would most always guarantee a P0420/0430 code. The ECU expects to see a near flatline signal if the cat is working properly.



A bad catalytic converter will typically result in a high waveform of the O2 sensor all the time. Below is a picture of the various waveforms and what you might see.




On the left is a normally operating cat. On the right, a cat that has deteriorated performance. Note that the A/F sensor reads pretty much the same either way. That is because it is upstream of the cat.


So how can YOU test the sensors and the cat so that you know what part has failed? You CAN test with a simple, quality volt meter, but it is not really easy. Using your Ultragauge or Scangauge, while giving you data, has a much too slow of a response for a valid diagnosis. This unfortunately results in the need for a diagnostic scan tool. Not everyone has access to this, thus the need for a good volt meter!

NEXT: I cover the process for performing the diagnostic process.

 

So you're saying it's 420 time? haha..

Good luck man..wife's Rav4 got that a few months ago, fortunately it was a gas cap and charcoal canister

EDIT: Never mind, hers was P0445

 

Waiting for mine to turn back on. Just saying I was getting it quite often, and I ran a bottle of MR gasket cataclean through my truck. It's been probably over 500 miles and hasn't come back yet. We'll see, and I'm subbed for your outcome

 

totally sub'd

 

What exactly is the P0420 code? The thread suggests guys like me to search. Cat trouble?

 

P0420 is "catalytic converter low efficiency" which in most cases results in the need to replace the catalytic converter. However some people have had luck with using cleaners. Also, seeing the P0420 is not always an indication of a faulty cat. Many times it is found to be a bad A/F sensor before the cat, or a bad O2 sensor after the cat. I didn't have time today to get into any of the diagnosis, but both yesterday evening as well as this morning I was on a couple of highway runs and the code re-occured. This means it was not a simple "glitch", but an impending problem. People have been asking me for a while to post my big discussion on the P0420 code for Toyota, and this means it is time.

 

Got it, thanks.

 

2nd post has been updated with information.

 

this sounds crazy but try cleaning your MAF. if the ECU is getting a false reading due to a dirty MAF it can throw a P0420 code. I went through this on my old truck after dropping over a $1000 on new cats and o2 sensors only to find out a $7 can of MAF/MAS cleaner did the trick. Hope this helps!

 

Thanks for the input. trust me, I am aware of the possibility of that. I make this thread because after so many people asking for help on this code, I finally ran into the code myself, and am explaining how the system works to help others understand it all.

 

Thanks for this info.. Been running with a P0420 code for quite some time and finally taking it to a shop friday as one of the o2 sensors is rusted off and dangling, not wanting to replace the whole exhaust so hopefully its a patch it up and replace maybe one sensor, will see if they try to sell me the farm, on a budget of about $300 so fingers crossed its not a cat and more than one sensor going bad

 

On a side note, is the cat absolutely necessary for the truck to run efficiently? My mpg have been dismal with the CEL, was thinking just hollowing out the cat or yanking it, but dont shops have regulations to follow and they wouldnt just weld in a straight pipe to replace a bad cat?

 

They won't. It's a big fine if they're caught doing it. It's illegal in all states, even for you to do it. Some states, like florida, just don't inspect or test

 

The cat helps with some efficiency, and to help reduce dangerous exhaust gases. Without it you would be polluting pretty bad. Some people just don't care, but some people are idiots. lol. I give a damn what my kids and grand kids have to breath. As for welding on a pipe in the place of a cat, it is a violation of federal law for ANYONE to do that, even to their own vehicle. Difficult to enforce it, but it CAN still be enforced.

 

Dont long tube headers and a y-pipe eliminate cats all together? so how is that legal if all states require cats

 

it eliminates the front cat (s), it still leaves a cat and if all cylinders exhaust are going through the cat I think it's legal.

 

Most states one cat is legal, but places like Cali it's probably not and I doubt the vehicle would meet it's emissions standards and wouldn't pass inspection deleting the front cat

 

Removing a catalytic converter on a modern car or truck is stupid. I mean without the cat you just dumping carbon monoxide into the air. So just leave your cats alone. Now if we were talking diesel emissions equipment I'd rip everything out last the cat and call it good.