Addressing some common misnomers of the P0420 code

As a member who sees fellow owners relying way too much on OBD codes to tell them exactly whats wrong, and as someone who gets paid to deal with these issues....

P0420 is catalyst efficiency, this is a logic code thats triggered when the activity of the rear sensor approaches the waveform of the feedback (front) sensor, typically 80% or more of the response of the bank 1 sensor (with newer wideband sensors, you cant see the waveform on a scanner, only the reference voltage supplied from the ECU).

The rear o2 sensor or P0420 code HAS NO EFFECT ON A/F MIXTURE OR ENGINE OPERATION. I have to type this in caps because so many people here attribute that sensor to poor MPG and the like, when the conditions that trigger it will also result in lower MPG and other drivability issues as a result of a totally unrelated condition. A P0420 code does not indicate whether the rear o2 sensor is or is not functioning properly. Correlation is not causation.

The P0136 however is a malfunction code relating to the rear o2 sensor. The o2 sensor is in essence a battery, the anode being the exhaust, the cathode being the reference air (this is why you never ever ever buy sensors you need to crimp to length, the reference air is drawn through the cable via a capillary effect). The semi-conductive substrate is designed to conduct what would be 0.5v when there is a 14.7:1 ratio of o2 from inside the exhaust to the outside. The ECU instead of supplying reference voltage, it is reading the changes in reference to its internal voltage.

Yes, your ECU is just a very complex and expensive voltmeter.



The varying level of oxygen entering the o2 sensor is going to vary the voltage level and tell the ECU what the exiting exhaust's o2 composition is, which in turn tells it if the catalytics are operating properly (we are only discussing the post-catalytic narrowband sensor, this does not affect the fuel control of the engine). There is an expected range that the ECU is going to be receiving from this sensor, if it is above or below that (damaged substrate or cracked housing), or the signal is intermittent (damaged wiring), it will turn on a P0136.

The catalytic uses o2 to recombine HCs and COs when in a rich condition, and COs recombine with Nox during a lean condition. This is why rich/lean switching is needed, a perfect 14.7:1 a/f ratio will not allow all three major pollutants to recombined. If the cat is functioning as it should, all excess o2 should be used in this process, thus the need for the rear o2 sensor to read near zero remaining o2. The ending result will be trace amounts of HC and Nox, near zero CO, and a majority of the output gasses consisting of harmless CO2 and water vapor (H2O).

If you have a higher end scanner with mode$06, you can see the raw hexadecimal data and the upper and lower ranges that would trigger a attached code if that said number falls out of range.

In conclusion....

The OBD scanner is a diagnostic tool, like a vacuum gauge or voltmeter, not a pocket mechanic. Ive seen way too many people on here that see one code and then rush out to throw money at parts without knowing what they are replacing. If you can learn to properly use all the tools you have as part of an overall diagnostic strategy, you'll save a great deal of time and money on your rig.

That should cover it.

 

Some solid info man.

 

OBD diagnostic scanners are a wonderful tool. I agree that they an end all and need to be paired with other tools and some knowledge/research.

While this is true the reason the code is set could have an effect on mileage, at least was the fact on my 2nd gen.

I was having trouble with this code and once I checked my fuel pressure I found the cause. My fuel pressure was low because of a dirty/plugged fuel pump. In turn the computer could not get enough fuel to the engine. The computer kept calling for more fuel and the O2 sensor was checking to see if it was there and it wasn't. Fuel trims vs O2 readings verified this.

 

Exactly, correlation is not causation.

Your situation was a perfect example of what I am trying to describe. You had an externally induced lean condition, not a catalytic efficiency issue, even though you had a catalytic efficiency code come up. Good work using the fuel trim data to confirm the issue. Thats the proper way to use scan tools.

 

For what it's worth and for anyone who reads this, I got the p0420 code. If I remember correctly the exact wording was related to the 1st 02 sensor. I cleared it to see if it would come back, which it did not. A little later I went to smog test and failed because my Cat Monitor wouldn't run. I did NOT want to buy a new cats, if I could avoid it, so I went through and did a process which I needed to do anyways. I cleaned the MAF using MAF cleaner, throttle body using TB cleaner, new coils and wires, iridium spark plugs, seafoamed in the vaccum lines, oil and gas, and that didn't fix it. I googled and searched TW for drive patterns to try to get the ECU to run the Cat Monitor and after 3 weeks or so of trying, no luck. I then went and swapped out both 02 sensors (from rockauto.com) on my Cali V6 and tried the driving tests again with normal day to day commuting and that was enough to get the Cat Monitor to run. The p0420 code has since never come back and I passed smog. Just my experience with it.

 

Read the OP again.

 

OP, Great explanation, especially about not using cut to length sensors.

 

I got what you were saying. I wish I had seen this a year ago though! Just my experience, lack of experience and what worked in the end.

 

So what causes a p0420 code? Are there multiple reasons? I've had the code for about 3 months maybe but just recently my truck has no power at all. It happened last month but went away in a day. The rpms are there but no power to the wheels. I'm not sure if both of these issues are connected

 

@Caligula

Thank you for the post

I had the 0420 bank 1 code about 2000 miles ago. It didn't come back . It is back now

Would you recommend swapping the o2 sensors with denso ones?

I don't have to smog for another 15 months

Since you are saying this code doesn't effect operation of the vehicle, should I just run it as is ?

Truck runs excellent, no other issues

Thanks for your continued knowledge

 

The OP describes the only possible reason for a P0420 code to exist. Other factors maybe be creating a condition that would cause the triggering of a P0420 code, though there is only one reason a P0420 would be present, which has been described in detail above.

 

While this statement is true, read here.

 

1st gen Denso sensors do not get reference air "thru a wire" in any way, shape or form. It is perfectly acceptable to splice a cut wire or use the terminate yourself (Denso) sensors provided the correct termination technique is used.

However, why anyone would buy a new sensor without the connector to save a few dollars is beyond me.

 

This post above is absolutely unequivocally WRONG. Please disregard it, the person does not understand what he is talking about. To the poster of the above quote, please do not troll this thread and attempt to spread misinformation.

 

Wanna bet................ again?

 

The ECU is very picky about receiving the exact waveform from the sensor, which is why it's recommended to use the OEM Denso sensor only. The addition of extra connections, longer length wire, etc could have an effect on the input to the ECU, however if you make high quality splices, it may not throw the signal off enough to matter.

 

Just the facts, Ma'am.

From here
http://densoautoparts.com/oxygen-af-sensors/oxygen-sensors

#2, PTFE filter



#2 PTFE filter seen thru the 4 holes.

 

Could of been my cat... experienced huge loss of power recently and my toyota found my cat was clogged. Have it straight piped for now power is back no code except for 0141 which when I put a new cat on well see if the 04200 reappears

 

So just my 2 cents... i recently bought my first truck and got a 2002 taco. Both the evap and cat systems were "incomplete" for OBDII. the guy i bought it from reset the CEL which i was oblivious to when test driving. Long story short... i couldnt even emission test the truck for a month and i was not very happy. I had to replace the evap canister and after talking with a few mechanics and friends decided to replace the upstream o2 sensor and front CAT. Now the EVAP system went ready and is solved but the CAT system still wouldnt go ready UGH! was getting P0420. Even though my intuition said it was the rear o2 sensor due to the data i was seeing with my scanner... my mechanic told me it wasnt likely the sensor since it was still sensing ... smh. Well i finally decided screw it ill replace the rear o2 sensor since its old AF anyway... well that solved it.

What i had been seeing for the sensor data was that the rear o2 sensor bounced around and my values for Rich to Lean was 129 with a max value being 128. So hindsight clearly shows that a value of 129 was showing that it was STUCK high but still showing SOME variation in data. Now my values are 28 out of 128.

At least now i have a brand new CAT, and new front/rear o2 sensors, new EVAP canister and solenoids. Feels so good to have fixed it all myself. Im a total newb with vehicles as im coming from an IT background. Im quickly coming up to speed on mechanic work though. i love working with my hands and LOVE fixing vehicles.

 

How difficult was the front 02 and rear 02 install?

I have been getting the on and off code for the p0420 and debating just doing both sensors together?

Where did you buy the sensors ?