Check Engine Light

P0420 OBD2 Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1)

In OBD2 Trouble Codes by 24 Comments

P0420 is a trouble code that gets stored in your car engine’s control unit when it recognizes a problem with the catalytic converter‘s efficiency.

There are many different reasons for this, so you need to know how to diagnose this trouble code properly.

Code P0420 Definition

Catalyst System Efficiency Below Threshold (Bank 1)

What does the P0420 Code Mean?

A P0420 Code means that the engine control unit recognizes that the catalytic converter’s efficiency is below the threshold.

The ECM uses two O2 sensors, one in the front and one in the back of the catalytic converter, to measure the efficiency. If the efficiency is low, the P0420 code will be triggered. In most cases, a P0420 code is caused by a bad catalytic converter.

P0420 Code Symptoms

You will probably not have any symptoms except the check engine light with just the P0420 code stored. You may have other problems with your engine damaging the catalytic converter, which can cause symptoms like rough idle, rough acceleration, misfires, and hard shifting. Always fix these problems first.

How serious is the P0420 code?

Low – The P0420 code will in most cases not create any further problems with your car’s engine.

The only thing that can happen is that the catalytic converter is so damaged that the catalytic converter parts come loose and blocks the exhaust pipe, which is pretty unlikely to happen.

The P0420 code makes your car’s emission bad for the environment, though, so you should fix it as soon as possible.

What causes the P0420 Code?

  • Damaged catalytic converter (most common)
  • Not Genuine catalytic converter
  • Wrong placement of the catalytic converter
  • Damaged upstream front O2 sensor / faulty wirings
  • Damaged downstream rear O2 sensor / faulty wirings
  • Exhaust leak
  • Intake leak
  • Oil burn (damaging the catalytic converter)
  • Rich/lean mixture (damaging the catalytic converter)
  • Misfires (damaging the catalytic converter)
  • A faulty engine control unit (rare)
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What repairs can fix the P0420 code?

  • Replace catalytic converter
  • Replace to a genuine original catalytic converter
  • Replace front O2 sensor
  • Replace rear O2 sensor
  • Repair faulty wirings
  • Fix oil burn
  • Fix misfires
  • Fix lean / rich mixture
  • Check the data with an OBD2 scanner from Amazon
  • Replace engine control unit (rare)

Common P0420 Code Diagnosis mistakes

The most common mistake is to replace the O2 sensors without doing any proper diagnostics. The cause of the P0420 code is most often the catalytic converter – which may get damaged by other problems with your car’s engine like misfires.

Bad O2 Sensors can cause the P0420 code, but is pretty rare.

Known P0420 Causes by Car Models

The P0420 code is more common in some car models than others. Here is a list of the most common causes per car brand. These car models are known to have a problem with the P0420 code. Remember that these are only general guidelines, and you should be making a proper diagnosis before replacing any parts.

1. Toyota Corolla

The most common cause when you find the P0420 on the Toyota Corolla is a bad catalytic converter. This can often be caused by oil going through the piston rings to get stuck on the catalytic converter if you have a Toyota Corolla that is struggling with the trouble code. Check for vacuum leaks and exhaust leaks first. Then check to see if you notice any blue smoke coming from the exhaust pipe. If so, it’s a sign that you might want to get expert help to find out where the oil is coming from.  A standard check is to check the crankcase ventilation.

If you do not notice any blue smoke at any RPM, it is most likely your catalytic converter is worn out.

2. Ford Focus

The Ford Focus usually has vacuum leaks or any broken solenoid, which causes a faulty air-fuel mixture and then causes the trouble code. Check your trouble code memory with a diagnostic scanner to see if you can find any trouble codes about the air-fuel mixture. If everything looks fine, check for exhaust leaks. Replace the catalytic converter if you can’t find any trouble codes or other problems with the air-fuel mixture.

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3. Subaru / Subaru Forester

The Subaru usually has the same problem that the Toyota Corollas have. Check for vacuum leaks or other fuel mixture related trouble codes. Check for any exhaust leaks before the catalytic converter. The most common problem with Subaru engines is the catalytic converter itself.

4. Volkswagen (VW) / Skoda / Seat / Audi A4 1.8T / V6 2.4

These VAG cars have some known problems causing the P0420 code. Check the check valves’ function under the intake and make sure the crankcase ventilation is free from dirt, causing the engine to burn oil, which clogs the catalytic converter. Check for exhaust leaks around any flex pipes on the exhaust pipe (common cause). Check for any trouble codes of the O2 sensors. If no problems were found, replace the catalytic converter. It’s a widespread problem on both the 1.8T and the V6 petrol engines. The 1.8T catalytic converter can be pretty difficult to replace if you do not have a lot of experience. The V6 has two catalytic converters, making sure that you troubleshoot and replace the catalytic converter on the right bank.

How to Diagnose the P0420 Code

  1. Connect an OBD2 Scanner and look for related trouble codes. Repair any related trouble codes regarding ignition or fuel before you try to fix the P0420 code.
  2. Check the live data to see the front and read O2 sensor signals. The car engine should be scorching – and the front sensor should fluctuate between 0-1 volts, and the rear should be steady at 0.7 – 0.9 volt. If it is not, there is a risk that the catalytic converter is defective.
  3. Heat the engine and check the temperature at the front of the catalytic converter and then in the rear. If the engine is hot and there is no difference in temperature before and after the catalytic converter – your catalytic converter does probably not work.
  4. If the catalytic converter is installed easily, it can be worth to remove the pipe from one end of it and check inside the catalytic converter for any visual damages.
  5. If everything points at a faulty catalytic converter – replace it. If you can’t find any problem with the temperature, voltage, or a visual inspection, you should try to repair other related trouble codes and then clear the codes and try again.
  6. If you still can’t find any problems. Make sure it is a genuine OEM catalytic converter, and it is installed in the original place. If everything seems fine – replace the catalytic converter.
Related:  P0132: O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)

Estimated P0420 Repair Cost

The estimated cost of repairing the P0420 code is the following. The prices are including parts and labor work at a workshop. The costs does not include diagnosis costs.

  • Catalytic Converter Replacement – 500$ to 1500$
  • Front O2 Sensor Replacement – 150$ to 300$
  • Rear O2 Sensor Replacement – 150$ to 300$

Common P0420 Related Questions

How to fix P0420 code?

To fix the P0420 code, you need to diagnose what is causing the trouble code. Start with inspecting and diagnosing your catalytic converter and continue with checking the O2 sensors.

What causes code P0420?

A bad catalytic converter is the most common cause of the p0420 code. This doesn’t mean that you should replace it, though. Always do proper research before replacing parts to save money.

What does P0420 mean?

The P0420 code means that rear O2 sensors report to the engine control module that the catalytic converter is not doing it’s job properly. The rear O2 sensor is comparing the signal from the front O2 sensor.

How to clear code P0420?

To clear the P0420 code you need to use an OBD2 scanner. Remember that just clearing the P0420 code will most likely solve the problem, you do also have to fix the problem.

24 thoughts on “ P0420 OBD2 Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1) ”

  1. This is the best article I have read regarding the damn P 0420 error code. Thanks for taking time to thoroughly explain the various causes and cures to this totally frustrating problem.

    1. Thank you! This article is awesome. It give total information for troubleshoot.

    2. Hey Magnus thanks for this great info I have a 2006 Honda Accord & this just popped up I’m gonna take it yo Honda but don’t want to get screwed any extra info would be greatly appreciated

  2. Hello dear brother Magnus greetings from pakistan i owned Daihatsu Mira 2015 model import from japan recently facing error code P0420 have checked the ceramic plate inside is 100% intact not broken even though not blocked but one thing i have doubt please clarify me the car bottom just near to bank2 second 02 sensor is got dent i guess may be hit with some solid stone or speed breaker etc is it also be a cause of this code. Thank u in advance

    1. Author

      It depends on how much it is a dent. If there are any leaks it is possible that it’s throwing the P0420 code. However, a small dent should not throw the code if it’s not restricting the exhaust passage a lot. It can often be a worn catalytic converter even if it is looking nice! 🙂

      1. Hi Magnus. Great page! I have one for you. My wife’s lexus throws a 420 code, it seems to happen during city driving only. When on the freeway and traveling long distances no codes. I took out the B1s1 and boroscoped the Cat No blockage noted. I also pulled the lower 02, it looked normal . After driving around town and at operational temp. I hooked up my tester and watched it on live, I did notice a spike or two on the bank one sensor 2, Otherwise it was a normal flat line. I was thinking about running Cat. Cleaner through it. I’m leaning towards a lazy 02 sensor. Do you have any advice?

    2. Very professional and informative article. It was very helpful.

  3. Mitsubishi outlander 2.0. Looks like faulty engine/coolent termostat triggered my p0420 bank 1 code..

    1. Author

      Yes, it can trigger the P0420 code if the engine is running very lean or rich which can cause the efficiency of the catalytic converter to get reduced temporarily. It’s not very common but it happens.

      1. Thanks for all the info, after a good reading i got a lot of information from this article, i was going to repair the code but like you said it keeps showing, and you mention that if there is oil leak it would be keep damaging the catalytic converter and thats what i been doing, replacing the catalytic every 6 months or so, and i have to keep an eye on my oil level because it keeps going down, thanks for all the info, i guess is time for a new car, old one gave me 10 to 12 years of service…

  4. I have a 2005 hyundai santa fe and I get check engine light with bank 1 and 2 catalytic converter fault codes only when I drive over 50 mph. I do a full drive cycle and the check engine light and codes go away. I also am able to pass the emissions test. I can not find any answers to this mystery, please help.

  5. Hi
    We have just bought a 2012 Speranza Tiggo 2 ltr automatic it’s not been used very often stood for long lengths of time as new condition it as been driven more than 800 km from when we bought it with no problem present but then the engine light symbol came came on we took it to a garage for computer check and DTC came up as P0420 description three way catalytic converter, 02 storage capability aging ( emissions out of range ) garage mechanic is saying it needs new catalytic convertor or he can drill hole in it which will do same job I live in Egypt so not sure about this advice please help 🤔

  6. Told no codes when bought 2006 JGC with 5.7 hemi. Now many past codes coming up with p0420. Mechanic said digital scan says not sensors. Auto zone now gave catalytic converter. Just had exhaust replaced for low intake code. Jeep dealer reset codes and knocked out both remotes. Should I try cleaner?

  7. Hi ! I have changed on my IMPALA 2001 3.8 : NEW EGR – 2 OXYGEN SENSORS – NEW MASS AIR – NEW SPARK PLUGS – NEW OIL – NEW AIR FILTER – NEW OIL FILTER – NEW FUEL PUMP – NEW FUEL FILTER – NEW GAS CAP – INJECTORS CLEANER – CATALYTIC CONVERTER CLEANER – and I still get the codes going from PO420 to PO171 …each time I have replaced a part !. I am very frustrated and I don’t know what else to do or change, I am 90 years old with LEUKEMIA and I need badly my car . Any suggestions, would be greatly appreciated . Many thanks in advance. 9/02/2019
    I have not said twice my questions ? After all those new parts the last code is PO420 !

  8. Hi Magnus,
    i have Toyota kluger motor with started smoking a month ago. the smoke was blue in colour , i was told it was rings which were gone as it was evident that the exhaust just at a touch , black like collection of oil was there. i had to buy , new rings , new spark plugs, new air filters new oil filters, fuel pump , and fuel nozzles , all were replaced but still the car is smoking. what could be the problem now as i am now frustrated.
    its a 1MZ engine and 6 cylinders

  9. Very informative article. I have toyota corolla 2003 with 1NZFE (1.5 lit engine). I have no codes and only problem is that the spark plugs are always reddish, just like rust colour. Replaced the plugs but same problem. My mechanic has told that this may be due to faulty catalytic converter as it is blocked. The car accelerates fine. Thanks Nadeem

  10. Had catalyst low threshold codes on BOTH catalysts = Bank #1 & Bank #2.
    2004 Acura TL V-6 3.2 , Automatic Trans.

    Fixed by installing spark plug anti-foul adapters between the O2 Sensor and Cat. They are $6 pair at Adv.Auto / AutoZone stores.

    You have to drill out (enlarge) the bottom hole in the adapters so the O2 Sensor can thread into it all the way. Very Simple with a drill and vise.

    On Bank #1 – actually had to use a second adapter to space the O2 Sensor out even further, to make that one stop throwing the code.
    It works – passes inspection — but realize this is a “grey area” — so don’t discuss it with your inspector.

    ALWAYS have your vehicle clean and neat inside and out before an inspection so that it appears well cared for. It can help prevent you from getting the full monte white glove examination top and bottom .

  11. Hi. I’m an older gal who does not know slot about this stuff. I just want to tell you, “Thank You” for this article. I learned a lot! Your information also kept a local shop from ripping me off, and for that, I am grateful!

    1. 97 buick century kids tried to reset key fob???? Using paperclip on data connection port 4-8 they said now no dashlights at all radio heat speedometer odometer everything gone fuel gage also please how do I fix this cheap

  12. Pontiac G6 2007. I’ve replaced everything and still after about 150 miles the check engine light comes on. Replaced all O2’s the CAT . Cam position sensors both of them fresh oil. No leaks. I’m at a loss. OBD code 420

  13. I have an exhaust leak in the manifold bank 1 that creating a ticking sound its quite when I start up the truck but as it gets warmed it the ticking gets louder I just replaced the upstream sensor but chose not to replace the downstream cause it wasn’t bad yet. I’m assuming either the exhaust leak or the the down stream o2 sensor is going bad or both are the cause I doubt my cats are bad consider it just past smog. Could you shed some light on the subject for me

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