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How do I fix diagnostic trouble code P0420 - Catalyst System Efficiency Below Threshold

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2005 Toyota Tacoma 4.0L v6

Catalyst system efficiency below threshold (Bank 1)

I have been getting this P0420 code off and on for several years.  I replaced my gas cap a few years ago and it seemed to go away for a little while (a few months) and the code came back.  I reset the code, mess with my gas cap, and the code seems to go away for a month or two and comes back.  It seems to indicate a bank 1 emission issue.  It seems like it could have been a gas cap but i cant believe it is a bad cap again.  Is it time for a new O2 sensor for bank 1?  Is there two sensors I would have to replace?  Which side is bank 1?  Thanks for any help

 

Mike

This topic was modified 2 years ago by ski4mike
97 Answers
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Posted by: @nchuffman

Is it possible that this sensor could bring up this code?

Yes.

Also, look for any leak in the exhaust system.

Thank you for the response!
I’ve checked the entire exhaust system several times, and placed my hand over the piping while the vehicle is running *while cold* have haven’t noticed any noises or stray air coming out.
I will be buying an OEM sensor on Monday when my parts guy gets back to the dealership, and I will try a factory replacement then.

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For P0420 start here:

https://www.obd-codes.com/p0420

https://mechanicbase.com/trouble-code/p0420/

https://m.youtube.com/watch?v=9VZ5K8n5jj0

https://m.youtube.com/watch?v=qJTmddbOxMk

 

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Check this out -

https://www.autocodes.com/p0420_volkswagen.html

The problem is, it's telling you the CAT is below efficiency but not what's causing it.  Try going thru this check list and hope for the best.

w8 you didnt get it . um sry i didnt say that right.. i dont have p420 now but the o2 sensors code witch sometimes its the upstream and some the downstream even tho i placed an adaptor - spacer.. read my first comend again plz.

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https://carkiller.com/scottykilmer/?wpfs=p0420

https://www.youtube.com/c/ScottyKilmermechanic/search?query=p0420

https://www.obd-codes.com/p0420

 

Posted by: @wahooo62

stays clears for 40-60 mi. then comes right back.

cat.con. may just be worn out

Maybe, but it’s only got 88K easy miles on it. As Doc pointed out there are no moving parts…. I’m trying to see if there is anything else I can look for or try before going the cat route. At least it’s pretty clean so I won’t be fighting a bunch of rusted seized up hardware. Famous last words….

I am thinking it’s related to burnt oil deposits that these engines are supposed to suffer from with the cylinder deactivation deal. Anyone got any thoughts there?

is it a flex fuel engine?

No it is not.

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Check carefully exhaust for leaks. A leak does not have to be significant, yet it will throw a code by upsetting an O2 sensor. Best way to do it is raise the car, start it, than ask a buddy to cover up an exhaust pipe with a rag firmly ( induce a back pressure ) and you - underneath - listen carefully for leaks all the way down to final flange of the cat. Even a smallest leak will hiss. If you find a leak, you have to eliminate it. If there is a leak, the cat is fine, … just a leak is a problem. Fix the leak, problem goes away.

Anyway, your car has lot of miles, and cat pretty well always fails because something else causes it. Oil consumption is always a problem, … so is mechanical failure ( check for damage - dent, cracks at the flanges, etc.) Check that. But don’t just replace your cat. Look for anomaly with the way the engine is running. Because if you don’t, whatever the problem there is, … and is not fixed,  it will kill your new expensive cat.

F.S.

Hello Figmund,

Thank you for answering. I can think where to check. When installing the downstream O2 sensor for Bank 1, I had to remove the exhaust from the catalyst to get my hand into the area to remove and install the O2 sensor. I looked for a seal and did not see any gasket. I have Honda replacements here. This is an easy check then.

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It is not normal. Did you check for any leak in the exhasut system?

I have to do an oil change this weekend so I'll do it then. I'll probably use the soapy water technique.

Late but I think the location of the bubblage is on the Resonator on the side where the lip is.

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Mostl likely either your catalytic converter is dying (likely on a 22-year-old car) or the O2 sensor monitoring it is defective.

https://www.fixdapp.com/blog/p0420-code/

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Bank 1 Sensor 2 is the downstream O2 sensor (i.e. after the catalytic converter) and should normally be a flat line around 0.5V (in general) while Bank 1 Sensor 1 is the upstream O2 sensor (i.e. before the catalytic converter) and should continuously fluctuate between 0.1 and 0.9 V.  Usually, if the catalytic converter is damaged the Sensor 2 readings will roughly follow the Sensor 1 readings, I.e. fluctuate up and down.

You say Bank 1 Sensor 2 flatlines at 0.035V, however, which is not normal.  Check the wiring, but also try swapping O2 sensors with the known good ones and see if the downstream sensor still flatlines as 0.035V.  If it does - then it’s not the sensor.

See P0420 troubleshooting below:

https://carkiller.com/scottykilmer/postid/82370/

Thank you for the response!
I figured that number was low, since Bank 2 Sensor 2 tends to flatline around that 0.5V mark, so I’ll buy a factory sensor and replace that Monday.

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I put my money on the 02 sensor having a failed heater element which is pretty common.  If the 02 sensor is malfunctioning, it won't be able to tell the computer the correct air/fuel ratio to put out and adjust the flow from the injector accordingly.

Thank you for the response!
I’ve noticed that the sensor has been having those weird numbers for awhile, and with the vehicle not running any differently, I’d wager you’re probably right. Definitely gonna be getting an OEM sensor and replacing that ASAP

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sensor 1 should oscillate. Sensor 2 should not.

Do you have them connected backwards?

I'm not sure. I know that nothing had changed and it had been good for the last year. But sensor 2 is oscillating and sensor 1 is not. That's also the case for my friends tacoma with the same engine.

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The two times I got a lone P0420 code, it was the cat that needing changing.

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If the CAT is actually failing, something else upstream is causing it.  Oil burning, excessively rich mixture, coolant being burned, malfunctioning egr or pcv valve, etc.

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I would be more concerned about the sudden loss of two quarts of oil than fixing the catalytic converter.  Odds are if you replace the converter that oil burning will foul up a new one.  Maybe do a compression test (or having it done by a mechanic) and take it from there.  2004 Xterras are supposed to be pretty decent vehicles so unless it's badly rusted out I wouldn't consider junking it just yet even with 225k miles.  Wouldn't put a lot of money into it either.

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well it's running too rich in that often will trip that code by making a catalytic converter not work correctly there are many reasons the car can run rich leaking injectors badly adjusted valves clogged intakes even a dirty air filter or fuel filter

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So the carne trying to lean out the air fuel mixture because it's running rich, hence the unburned gas smell. Can probably inspect to see if there be a leak in the exhaust but my bet is the o2 sensors though. If not then it could be bad gas that is causing the cat to fail prematurely.

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Well you're 1,000 miles over. But it's still worth a shot.

From your owner's manual:

Screen Shot 2021 12 03 at 13.24.50

Thanks for the info Joe when I purchased a car from carmax I did get extended warranty so maybe that will pay for a new cat.

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Low hanging fruit question:  Did you replace the O2 sensors with OEM?  And which ones did you replace?  (Upstream, downstream?)

See link for further troubleshooting (will need a scan tool):

https://carkiller.com/scottykilmer/postid/82370/

Sorry for the late post I did use OEM bosch.

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It is most likely the sensor. 

You can also crawl under the vehicle and knock on the catalytic converter. If it sounds like marbles, the internal baffling is shot and you need to replace it.

Just ne sure to perform this when the vehicle is cold to prevent burns.

Thank you for the response!
I actually did that a little while ago, after I took the vehicle out for the drive that I took some of the live data from, after it had cooled down, of course. Other than the pipe shaking, I didn’t hear any odd sounds, so I would be pretty confused if that cat ended up being bad. Gonna buy a factory sensor Monday and swap that out, hopefully it helps!

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02 sensors and bad injectors

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It's where the exhaust exits the engine. Maybe this will help, the locations of the sensors is explained at 8:25.

 

 

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Have you searched Scotty's youtube channel?

https://www.youtube.com/c/ScottyKilmermechanic/search?query=p0420

 

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If you have the P0420 the converter is most likely reached it’s end. Cleaner is a gamble on if it helps or not but I think you’re beyond that 

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I would first find out what is causing the P0420 code.  Is it actually a failing CAT on an 18 year old engine, or is it running rich due to old warn out fuel injectors, or burning oil past worn out piston rings.  I would start by finding an honest mechanic who will do a compression test and clean the injectors before spending a ton of money on CAT, only to have the new ones clog up because of another problem.

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With that mileage, the cats likely are culprit.

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Some model manufactures do require their cats as universal will not work.  It's coding in the ECM/PCM and not a way around (legally).  First, I'd get gas somewhere else and higher octane to see if that makes a difference.  Sometimes a tad better gas will allow non-manufact. cats meet the programmed efficiency.  If that didn't work, hook up your ODB2 and live monitor/record O2's, LTFT and maf while driving. Hopefully something gross will pop up but you just might be stuck with buying new cats.  Unfortunately a common problem that's been around for decades.

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If the above solution did not work, check the Oxygen sensors to check if they are working properly. If they are fine, clean the cat using detergent (check out Scotty's video on that). Good luck.

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I did an Emission check on Thursday and on Friday I did fill the tank with gas. Saturday Morning after 10min drive, I see a check engine light. On Scanner Tool it says PO420 Code. Can someone please help me with what to do with this issue?

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I would not bother with getting a used catalytic converter. Just bite the $400-$500 bullet and put a new one on. Then you can go for the life of the car. Just not a part to be cheap on and then pay again in 30k miles.

 

Either one could do the job, but I would go muffler shop. Their rates are usually cheaper than mechanic rates and they have all the equipment they need to make the repair right.

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When they go they go. Not much else you can do but replace it 

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