P0420 – Meaning, Causes, Symptoms, & Fixes

Code P0420 Definition

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 during the combustion cycle and convert them into harmless gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by the vehicle.

P0420 Symptoms

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

 P0420 Causes

  • Faulty oxygen sensor
  • Worn and internally failing catalytic converter
  • Leak in exhaust system

Code P0420 Severity – Moderate   

While P0420 does not pose a danger to the driver and may not produce any symptoms with issues related to drivability, it can cause severe damage to other components 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.

Code P0420 Common Diagnosis Mistakes

Do not overlook other codes that are paired along with P0420. Codes P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0174, and P0171 can give false data readings that create conditions which trigger code P0420. If any other codes are present, they should be addressed first.

 P0420 Diagnosis

Tools Needed to Diagnose:
  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 car 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.
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Comments 43

Joe Manislas

March 6, 2018

This was very informative!

William Reed

April 8, 2018

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.

Mason Mayes

April 11, 2018

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

Dale Fey

June 11, 2018

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

Daniel Akpan

August 28, 2018

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


October 11, 2018

Where Can you find bank one on the catalysti converter

gordon flash

April 6, 2018

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

Mason Mayes

April 11, 2018

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 https://www.amazon.com/Jeep-Grand-Cherokee-Gasoline-engines/dp/1620921073/ref=sr_1_1?ie=UTF8&fixd_blog-20&qid=1523413703&sr=8-1&keywords=2011+jeep+grand+cherokee+repair+manual


April 6, 2018

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 ?????

Mason Mayes

April 11, 2018

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.

Wynell Hansmann

November 13, 2018

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.


April 6, 2018

Thank you this was very helpful understanding how things work.

Mason Mayes

April 11, 2018

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

Bob Silva

April 7, 2018

Worked like charm

Cruz P Matias

April 8, 2018

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

Cruz P Matias

April 8, 2018

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.

Mason Mayes

April 11, 2018

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!

Enrique Cepeda

April 10, 2018

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

Mason Mayes

April 11, 2018

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: https://axleaddict.com/auto-repair/How-to-Check-an-Oxygen-Sensor

Jeff Kohlmann

April 14, 2018

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

B. Dolmovich

August 7, 2018

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

Bob Allen

April 23, 2018

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).

Glenn Gibson

May 25, 2018

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

Mason Mayes

May 25, 2018

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


May 25, 2018

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?

Mason Mayes

May 29, 2018

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.

Mason Mayes

May 31, 2018

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!

Frank D Brown

May 26, 2018

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.

Mason Mayes

May 29, 2018

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

Bob Allen

June 9, 2018

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?

FIXD Marketing

June 25, 2018

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!


July 2, 2018

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 )

FIXD Marketing

July 9, 2018


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


July 14, 2018

Thanks. I understand the code much better now.


July 14, 2018

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

FIXD Marketing

July 18, 2018

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

Olga Layto

August 19, 2018

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?


September 6, 2018

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


September 14, 2018

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

Subie Doo

October 18, 2018

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 😉


November 7, 2018

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


December 13, 2018

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

FIXD Marketing

December 13, 2018

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