Auxiliary Emissions ControlOBD2 Codes

P0420 – Meaning, Causes, Symptoms, & Fixes


Code P0420 Definition

Catalyst system efficiency below threshold (Bank 1).

P0420 catalyst system efficiency below threshold bank 1

What Does P0420 Mean?

The function of the catalytic converter is to break down harmful pollutants that are created by your vehicle during the combustion cycle and convert them into less harmful gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by the vehicle.

What Are the Symptoms of Code P0420?

  • Check Engine Light is on
  • Lack of power from the engine
  • Decreased fuel economy
  • Rotten egg or a sulfur smell

What Is the Cause of P0420?

  • Faulty oxygen sensor
  • Faulty air-fuel sensor
  • Worn or internally failing catalytic converter (Most Common)
  • A leak in the exhaust system
  • Misfire (the root cause of converter failure)
  • Rich or Lean air-fuel ratio (the root cause of converter failure)
  • Leaded Gasoline (the root cause of converter failure)

How Serious Is Code P0420? – Moderate 


While P0420 does not pose a danger to the driver, it can cause driveability concerns such as a lack of power or poor acceleration if left unrepaired. It can also cause severe damage to other components of your vehicle if not addressed in a timely manner. It is recommended that you address P0420 as soon as possible to keep the cost of the repair to a minimum.

Common Diagnosis Mistakes for Code P0420

Often times this code is thought to be a faulty O2 sensor or A/F Sensor; while this is a possibility, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired along with P0420. Codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are misfire codes, and these codes require misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will go bad again. Also, make sure you don’t have codes P0174, P0171, P0172, or P0175 which can mean the engine is running rich or lean, which can burn out your catalytic converter. If these codes or any other codes are present, they should be addressed first.

Tools Needed to Diagnose Code P0420:

How to Diagnose Code P0420:

Difficulty of Diagnosis: 5 out of 5

  1.  Scan your vehicle to verify P0420 is the only code present. If other codes are present, they must be addressed first.
  2. Examine the exhaust system for damage and leaks. Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that was the fix.
  3. With the vehicle running and at normal operating temperature, use the digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.

Estimated Cost of Repair

For error code P0420, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.

  • Air fuel sensor or oxygen sensor $200-$300
  • Catalytic converter $400-$2400
  • A leak in exhaust $100-$200 (if welded to repair)

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  1. This was very informative!

    1. Bank1, sensor 1 was replaced on the 7th of April. Electronic load detector was replaced about 2 weeks ago. These two things were detected by my cousin’s FIXD device. I did the ELD
      myself a friend with a lift did the sensor.

      1. Hi William! Thank you for sharing your experience with us! We really appreciate the feedback!

        1. I went through a huge puddle of water after that check engine light came on. We tested it and p0420 code came up.we cleared the code no return of check engine light. What was the problem the water shocked the cat and sent a signal? Need an opinion

        2. Live in a cold climate and noticed that when extremely cold that this code comes up. Is there need to worry? It has come on and off.

          1. Not to worry unless the CEL stays On.
            Cold weather brings out the Gremlins in the 1st gen Tacoma, lol
            Everybody has them!

      2. Both my vehicles one a 2003 GMC Envoy SLT and 2009 Nissan Murano SL. Have the infamous P0420 Code.
        I want to know what kind of money iam looking at. To late for the GMC but my recent car that I got on April 16, 2019 two days later the code pops up
        What’s up with that.

        1. The seller probably cleared the code just before the sale, and it took 2 days to reappear.

        2. I had went to put the Catalyst converter so why is it showing up if it was done just my check engine is on that’s what’s the problem yes I like to get it fixed

          1. Can the converter be bad and not have any of the symptoms listed? I’m getting the cose but no issues with rotten egg smell, gas mileage is same as before…etc.

          2. Shannon, if you arent getting the code yet, i strongly suggest adding cataclean or some kind of cleaner, (CRC guarantee too pass is good as well) while you still can. Once it’s completely clogged, the cost can be astronomical. (for my 2011 JEEP LIBERTY its $2400 to replace) and only $200 of that is labor. Some states (NY being one) enforce what’s called the “California emissions” requirement, which means you can only use the converters supplied by the dealer, and designed to pass California emissions.

      3. What is the actual cost of a catalytic converter that the suggested parts and repair is estimated btw $1500 & $2000?

        1. I just did mine, 08 Malibu. Paid $100 for the part on Amazon and $250 to install… Was quoted much higher.

          1. Reason is because this is a general cost if you’re having the garage purchase and install. Also keep in mind that more states are going to California emissions which is quite pricey which is rather inflated pricing and dealer only for emissions parts

        2. I get all my parts from I only paid not even $280 for bank 1 cat and four 02sensors.

        3. Look on card to look for parts

    2. Yes I did know you had trained service professionals who could assist me with advice.

      1. Good morning sir, pls I want know more about the fuel consumption in a particular vehicle what cant be the cause of that after putting all the catalyst and oxygen sensor

        1. Very good information . I’m very grateful .7

      2. Hi,
        I just got my Bank 2 catalytic converter replaced in April so I would pass emissions and now my bank 1 catalytic converter is out? Is that possible? Should I have changed all together if this was going to happen?

        1. How do I find the videos on fixing the car it shows the parts

        2. I always buy my parts and labor is cheep where I’m at in ky it’s 70 to 75 an hour I get most of my parts at there a great place even have dealer parts also

      3. Awesome tool

    3. Where Can you find bank one on the catalysti converter

      1. Where can I find bank 1 on 2003 Honda accord ex

      2. Is it worth it changing the catalytic converter on 2005 Toyota Camry

      3. I have a 2007 Nissan Maxima. I have a p0420 code. I just replaced the bank 1 catalytic converter and both O2 sensors and both Cam shaft position sensors. I thought the problem was fixed but after performing along test drive, I am getting hesitation when I try to accelerate from a dead stop and the code returned along with the check engine light. Once it gets passed 2000 rpm and in second gear it accelerates like normal. This hesitation only seems to be present after the engine warms up. It didn’t happen until the end of a 40 mile test drive at highway speed. Any ideas?

      4. I need some help. I have a 2007 GMC Acadia. The engine light is on with a P0420 code. This is what has been replaced/repaired
        1. Mass airflow sensor
        2. Throttle body
        3. Valve covers/gaskets
        4. 2 upstream O2 sensors & 1 downstream sensor.
        5 . catalytic converter has been replaced twice
        All of this and the light is still on. Can some one please give me some advice on what to do now. I don’t know if it is part of it, but the evap code always comes on & won’t reset. Please help!

        1. Based on the instructions given here, you need to concentrate on that EVAP code before worrying about the P0420 code. This code has shown up on my Envoy and what the EVAP is or what the EVAP problem is -that the only one I haven’t figured out yet concerning this problem.

        2. Evap Code most likely is you need to get a new gas cap. Also, make sure the area around the gas cap is free of rust etc.. Hope this helps, so many times people go through so much when it’s just the gas cap.

        3. Spray some wd40 in the back of the engine while its running. If it starts to die down or shut off, you have a vacuum leak

    4. i have a 2004 Z06 and removed the secondary air pump along with all the tubing which supplies fresh air into the headers for cold starts during only the first 3-4 minutes and after 2 years im guessing the ECM doesnt know its missing cause its been running like new since then with no issues

  2. where is the camshaft position sensor bank 1 located in the 2011 jeep grand cherokee

    1. Hi! It looks like it should be above the water pump and behind the accessory belt. We highly recommend getting a vehicle specific repair manual that provides diagrams of all of this, such as

      1. I changed the crankshaft sensor the catalyst system efficiency below threshold b1 show up the engine light went off. Do I need to change catalyst converted to

  3. I had a new catalytic converter put in the automobile. I have taken it to the garage prior to purchasing the FIXD product. Garage said that the test they ran says their is a short somewhere and that is what is causing the error message. Converter is working fine according to mechanics. To find the short, they said that they would have to test virtually every wire around the engine, and the labor to find the short could cost hundreds of dollars. Rather than pay 100’s of dollars strictly in labor costs, I have chosen to ignore the engine light. Interestingly, after I use the Fixd to diagnose the engine, it then resets the engine light to off. Usually within a short time, or a few miles of driving, the check engine light comes back on. And your recommendation is ?????

    1. Hi Mike! If the car’s engine control module recognizes a fault after the check engine light is reset, it will cause the check engine light to come back on.

      1. How do we fix the engine control module? Mine is doing the same as Mike’s. I have replaced all the sensors and a new catalytic converter.

  4. Thank you this was very helpful understanding how things work.

    1. Hi Leon! Thank you for the feedback, it really means a lot to us!

      1. Thank you for sharing, it was very informative and somewhat a relief from worrying.


      1. Now that is a great question, Armando

  5. Worked like charm

  6. p1272 code
    replaced the following on a 2005 Altima. both up and downstream sensors, and the exhaust manifold with catylitic coverter also replaced MAF sensor and the fuel pump reset the code day later thecode came back. it said engine running too lean the only thing I didn’t replace is the throttle body
    any suggestion, i

    1. I have a 2005 Nissan Altima with code p1272 . which I’m told is running too lean I replaces the exhaust manifold w/caty convertor. also replaced both up and down stream 02 sensors , MAF sensor and the fuel pump, cleared the codes with FIXD day later same code came back the only thing I didn’t replace is the throttle body fuel has a internal filter. so I thought the fuel pump pressure might be the cause. any suggestions.

      1. Pvc valve replaced?

    2. Hi! Thank you for reaching out. Codes that start with P1 are manufacturer specific. Could you please provide us with your year, make, and model and we will look into it!

  7. Would you happen to know where I can put my multimeter to test the voltage? Is it on the o2 sensor or the connector?

    1. Hi Enrique! You connect the red probe to the signal wire, and the black probe to a ground. We are working on producing how-to’s with this information, but in the meantime, we found this to be very helpful:

  8. Manifold leak fixed but light keeps turning on. Not worth $1,500 for new cat. Light stays off sometimes weeks, sometimes less sometimes more.

    1. Try a local muffler shop. Mine had generic (universal?) catalytic converter for about $300, installed.

  9. I have a 2002 PT Cruiser getting this error. It has only one oxygen sensor an it is upstream of the converter. How do I voltage test the O2 sensor (BTW I replaced the sensor and I still get the error).

  10. Used a gas treatment, light out after 16 gal. Time will tell if this fixes the problem

    1. Good luck, Glenn! Please let us know if it does!

  11. I have FIXD and I got P0420 with Check engine light, VSC and Trac Off lights on the dash board. What is the relationship between P0420 and the VSC and Trac off? why are all three warning lights come on. I reset them with my FIXD, the lights go away but they appear after less than 100 miles. I noticed also when I turn teh ignition key on without starting the car, there is a hissing and clicking sound coming from behind the brake reservoir. How do all these problems combine to give this P0420?

    1. Hello Emilio! We are sorry about the troubles you are having with your vehicle. I passed this along to our team and they said they would go through the diagnosis for P0420, fix it, and then see if the the VSC/Trac Off lights go off.

    2. Hi Emilio,

      Our team did some more research and found that if your catalytic converter is severely clogged, it could restrict the exhaust which would impact the calculation of engine load, thus causing the VSC and Trac Off systems to stop working. Please let us know if this helps!

    3. So I’m not sure what’s going on with mine, it’s the same code but for some reason my check engine light will turn off by itself but then a few weeks later it comes back on with the same code. The oxygen sensor has been replaced twice, I have no leaks, the only thing I haven’t done is replace the catalytic converter itself, but I checked the downstream o2 sensor and it’s reading 0.45v it’s just driving me nuts that the light won’t stay off

  12. I replaced the engine in my 95 Ford Mustang. With the higher compression from the engine pushing out old carbon build up, I received this fault code.. 1 can of Sea Foam cleared it up. I had to reset the computer 4 times.. thanks to Fixd.. quick and easy to do.. car is running like a champ.

    1. Hi Frank, thanks for sharing your success with us! That is awesome!

  13. I have a 2002 PT Cruiser, there is only 1 O2 sensor and I have changed it to no avail. Any suggestions on what to try and/or how to diagnose the issue?

    1. Hi Bob,

      Check to make sure that the readings with your multimeter are correct. This will verify that your catalytic converter is functioning properly. If this does not fix the issue, contact our Mechanic Hotline to gain more insight!

  14. I recently purchased a used 2008 Saturn Vue and my check engine light is on and showing code P0420 catalystic system failure because of an exhaust leak, where would I find my exhaust and what am I looking for? ( I am a young female and know nothing about cars )

    1. Hi!

      You will have to reference your car specific service manual to locate your exhaust and other car parts located under the hood.

      The FIXD Team

  15. Thanks. I understand the code much better now.

    1. I just replaced both 02 sensors in 2002 Yukon XL 1500 car seem like it was fixed then it shutoff same issue persists what else could it be ?

  16. I have replaced the catalytic converter twice and still have the same problem

    1. Hi Carlos,

      The catalytic converter is only one of the things that could be problematic in trouble code P0420. Please do not overlook other tests and possible errors that it could be!

      Thank you,
      The FIXD Team

  17. Hello! My check engine light (solid) and cruise control (blinking). FIXD code detected was P0442 – Evaporative emission control system leak. I had FIXD clear the code prior to taking my 2005 Subaru Forester to a mechanic. The engine light and cruise control lights went out. Today a week later, the lights are back on. I used FIXD and Code P0420 – Catalyst System Efficiency was detected. I’m assuming that by not addressing PO442 code it lead to Code P0420?

    1. I have a 2009 Suzuki SX4 with a code P0420 and I am trying to figure out which one to change can you help me please

      1. I want to find out if it is bank 1 or 2 censor up stream or down stream

  18. Oi Bom dia estou com o cod P0420 ativo na minha XC60 2011 e tambem esta ativo o cod P0171.
    Alguma dica.

  19. I put my Bank 1 Sensor 1 O2 sensor on yesterday and today engine light came back on and I did a reading and it say P0420 catalyst system efficiency below threshold bank 1. Can anyone tell me what it could be

  20. Would a modified exhaust system cause a P0420 Code? For example a “Cat-less” Exhaust System that is not OEM but all direct bolt on (no welding).
    This is for a 2000 Subaru Impreza 2.5RS – Manual. Exhaust System: Spec-D Headers (Heat Wrapped), Stromung Catalytic Eliminator Front Pipe with Bosch Upstream and Downstream O2 Sensors, Borla Intermediate Pipe and Muffler. All installed on 3/18 including Gaskets.
    The reason I ask this question is because with this type of Exhaust System, I am getting that code P0420. All brand new parts, all installed at the same time, all with new gaskets, and with the bolts torqued to spec. So that would eliminate “leaks” as the cause. The O2 sensors were also installed at the same time. So that would eliminate “bad O2 sensors” as the cause. Now for the Catalytic Converter…there is really none to speak of because of the Stromung Catalytic Eliminator Front Pipe 1999-2005 Impreza Non Turbo I also installed at the same time. It is a direct bolt on replacement piping, specifically designed to replace the catalytic converter. Were I live there is no emission’s restrictions/requirements and I mainly use it for off road use (track car).

    So is that the reason why I am getting that code? The only other thing I can think of is the wiring because of the year and age. Any ideas would be greatly appreciated and thank you for reading my novel 😉

    1. I don’t know if you ever solved this issue, but for you and all our readers,; you have to keep in mind that without a cat, you’ll get basically the same signals as the upstream sensors. The computer sees this as a failed cat and sets a P0420 code. What the computer wants to see is an almost steady signal of around 0.45 volts. While some computers can be ‘fooled’, most are set up to set an error code if you attempt to circumvent the downstream sensor. Doesn’t hurt to try!! Good luck.

  21. Where would the catalytic converter bank 1 be on a 2006 Chrysler Sebring convertible?

  22. where is the downstream O2 sensor located in the 2007 Honda Accord. I need to measure
    the voltage reading of the downstream O2 sensor.

    1. The downstream O2 sensor should be located post-catalytic converter.

  23. My catalytic converter pass state inspection but it’s say p0420 bank 1 because of it my service light won’t turn off I also been told that my service light and check engine light are different

    1. Hey Jeffry! The check engine light typically means there is something wrong with your vehicle, while the service light can simply mean it is time to have your car serviced (i.e. get oil changed or rotate tires).

      FIXD Team

  24. Would you recommend trying a catalytic converter fuel and exhaust system cleaner to address a P0420 fault? If so, what is a good product/brand to use?

  25. When I scan the 2005 GMC Safari, with “live gauges”, the #2 and #5 O2 sensors (One is upstream, the other down) are jumping all over the place and it sets a ‘420’ code about once every 2 or 3 weeks. Bad O2 sensor? A weak spark plug? Thoughts?

  26. I have a 2006 VW Jetta 2.5L with CEL on. The code is P0420. I did a temp reading at Cat Converter. The exhaust side is 150+ higher in temp. There are no other codes attached. Any Idea?

  27. For this code p0420 if I wanna fix it, by priority, which one should we fix and replace: oxygen sensor or whole cat?

  28. Witch side is bank 1 on a Nissan Extera

  29. I have a 2007 Nissan Xterra, only code it’s showing is P0420 catalyst system efficiency below threshold bank 1. Did some research and some people are saying it’s the catalytic converter is bad. Any ideas?

  30. I have a 08 Chevy Silverado v8 I have 4 O2 sensors which one would be Bank #1 if someone could help me out with this question be much appreciated. Thanks

    1. Bank 1 is drivers side. Upstream is before the catalytic converter. Downstream is after the catalytic converter. Bank 2 is passenger side. Upstream is before the catalytic converter. Downstream is after the catalytic converter.

  31. I replaced the upstream O2 sensor on my 2010 Chevy Aveo, but still have the CEL and P0420. Would not replacing the downstream cause this?

    1. Possibly. It’s also as likely that you simply have a bad distributor cap, or a coil pack that’s got a weak output. Even a spark plug that’s worn can cause this code. Many times you can see it on the “Live Data” of the fix’d app. This is especially true if it happens most often after driving in the rain. Moisture can get into the distributor or the coil pack and cause an erratic spark or signal. Check those! Loose exhaust connections can be tricky to find, but at idle often sound like a chick- chick- chick- sound. The reason for this isn’t that the exhaust comes out, but that between each cylinder’s individual exhaust there’s a slight vacuum created that draws in air and throws all the readings off, causing the computer to set the code.

      1. Wow that is great. I had a P0300 P0301 P0304 all mis fire. Then driving in rain threw P0420. So just what you said. So what is it just coil pack? I just bought new plugs and coils. So what is it?

  32. Have P0430 and the car with skid marks warning. I just took my SUV out of shop to get rebuilt transmission. Could these warnings have anything to do with rebuilt transmission?

  33. My FIXD is showing a problem with the catalytic converter (Bank 1). At the dealership they verified the issue. However I got that converter replaced last October at a garage in my town. I cleared the check engine light and 3 days later it came back on. Guess I have to take it back to the garage where I had it worked on last year. SIGH!!!

  34. FYI, never had an error code before until I added a fuel treatment from Lucas. The engine ran fine, no problems until the engine light came on sometime after adding the treatment. Cleared the code and it has not come back since, but I haven’t added anymore fuel treatment either. The fuel treatment has not made any of my other cars engine lights come on, just my GMC Sierra 1500.

    1. You didn’t say which code you got, what year, or how much mileage. It’s possible that the treatment dislodged some gum- AKA “crud” that deposited itself on a spark plug or the O² sensor. Many times this deposit will burn off and not be a problem afterwards.

      1. My 2003 Chevy Avalanche is throwing both P0420 and P0430. With the advanced troubleshooting on the FIXD it is telling me that the evap system is failing. I have replaced the vent valve and the purge valve and the gas cap. What should I look at next?

  35. Does anyone know of code P420 on a Tacoma can be fixed with gas cap replacement?

  36. I found the info quite useful but would like to explore much more.

  37. code p0420 Three months ago light came on and i replaced the o2 sensor and light went off for 3 months. It has now come back on with the same p0420 code. I did not replace the a-f sensor where is it located and what are my options?

  38. I had the P0420 code pop up a while ago. I changed the downstream O2 sensor, which didn’t solve my issue. A few days ago I changed the bank 1 catalytic converter. When I turned the car on and ran a scan the P0420 code along with 5 out of 6 cylinders showing a misfire. I also saw the sensor needed calibration, which I did. The misfires didn’t return but after I cleared the P0420 code it returned a couple days later. I guess my next moves are to change the upstream O2 sensor and/or checking the new catalytic converter for a leak because it was a pain in the neck to install.

  39. Has your AC has been recharged in the last 24 hours? Is so you may get one P0420 on the next startup. The gas is heavier than air, gets sucked into the intake manifold .. and fools the O2 sensor into believing the catalyst has failed.
    Reset the MIL and the problem will not recur.

  40. You should have been resetting the check engine light while making those repairs, but if you didn’t, go ahead and reset it with a FIXD Sensor or by disconnecting the car battery for about 10 minutes and then run the car for 50 to 100 miles. If the light comes back, check your codes again, if it doesn’t come back, the problem is fixed and your scanner should read Ready.

  41. I have a subaru forester 2007 non turbo that has a CEL for P0420 (bank 1). I’ve used a OBDII scanner, and it shows that the short term fuel and long term fuel trim in bank 1 are ok, but shows a result of 0 for bank 2. Is it possible that the downstream sensor is at fault? Even when the OBDII scanner says bank 1?

  42. Can a P0420 (bank 1) be caused by the downstream 02 sensor (bank 2), or is it only for upstream (bank 1)?

  43. You are an awesome tribute

  44. 2014 Chevy Cruze p0420 code keeps coming back on I just had catytic converter put on. 02 sensor are good so what bow

  45. 2013 Chevy Equinox FWD. Just drove 1300 miles from Cal to Washington . Vehicle has 78,000 miles on it. Running fine, no issues great gas mileage 28-30 mpg. Car had California Smog test about 4 months ago and tested fine. Today while driving check engine light popped on, no other indication of any problems. Pulled over, and ran a FIXD diagnosis and P0420 was code. No suffer smell or rough running. Checked gas cap and it was not fully to lock (clicked) position. Secured it and reset engine light and so far it has not returned. Could not having gas cap secure account for this reading or do I need to go to the dealer. Catalytic Converter is supposed to be covered repair under warranty for 100K as I recollect will check manual in the AM before going to dealer if light comes on again.. If I keep driving for awhile with no other symptoms and check engine light does not return am I likely in the clear? I seem to recall that if gas cap not secure that can effect emissions.

  46. Can a bad catalytic converter after having put a hole in it to unblock it cause your air conditioner to turn off while driving the car ? Then after shutting the car off for while and starting the car again the air conditioner will work again.

  47. Check the coils. I have an ’06 Dodge Dakota that was getting P0420 codes. I found the spring inside the #1 coil had disintegrated, causing a weak spark and a rich fuel mixture in that cylinder.

  48. I have replaced both o2 censors on my 02 Camry no check engine light is on but still telling me that it is the problem don’t know what to do

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