Question about 2004 Mazda 3

2 Answers

P0420 I have code P0420. I connected a scanner to check o2 sensors and front and rear have similar readings. O2 sensors are ok? I need to change catalityc? What another things could check to solve the problem?

Posted by on


2 Answers

  • Level 3:

    An expert who has achieved level 3 by getting 1000 points


    An expert that got 20 achievements.


    An expert that got 10 achievements.


    An expert that got 5 achievements.

  • Mazda Master
  • 14,417 Answers


Posted on Apr 02, 2014

  • kzone10 Apr 02, 2014

    Pep Boys and other parts stores will perform a code diagnositic for free.


  • Level 1:

    An expert who has achieved level 1.


    An expert who has answered 20 questions.


    An expert that has over 10 points.


    An expert whose answer got voted for 2 times.

  • Contributor
  • 38 Answers

Hi Osar-
I pulled this link from It lists the possible causes, diagnostics and resolution for engine code issues. Hope this helps!
P0420 OBD II Trouble Code

Posted on Apr 02, 2014


6 Suggested Answers

  • 2 Answers

SOURCE: I have freestanding Series 8 dishwasher. Lately during the filling cycle water hammer is occurring. How can this be resolved

Hi there,
Save hours of searching online or wasting money on unnecessary repairs by talking to a 6YA Expert who can help you resolve this issue over the phone in a minute or two.

Best thing about this new service is that you are never placed on hold and get to talk to real repairmen in the US.

Here's a link to this great service

Good luck!

Posted on Jan 02, 2017

  • 2028 Answers

SOURCE: Mazda mpv O2 sensor wires?

Put some penetrating oil on them and let it soak overnight. This will break up the rust and other build up. Make sure that it is penetrating oil, you can get it at any automotive store.

Posted on Jun 28, 2008

SOURCE: O2 sensor

There should be two O2 sensors. If you look inbetween the engine and radiator following the exhaust manifold down to the exhaust pipe you should run into the 1st O2 sensor, following the pipe down stream under the vehical towards the catalytic converter you should find the second O2 sensor. The 1st O2 should be easy but the 2nd might be difficult depending on your abilities. Coils will depend on 4 or 6 cyls, 1 for each cyl. Repair man, go to, pep boys, napa, auto zone, advanced auto parts to name a few.

Posted on Mar 18, 2009

  • 80339 Answers

SOURCE: Fault code P0133 can't pass inspection.

This code does not always mean the O2 sensor is low, but if u want to replace it here is the info to do that and the part number.

The sensor you want is the one in the exhaust pipe of the engine cylinder bank that has #1 cylinder in it, as you face the engine with the hood open the cylinder bank u want is the rear bank, u will need to jack the car up and get under it to remove the sensor, u may also need a special socket to remove it, the sensor is the one that is before the catalytic converter. part number is:DENSO Part # 2344068
REAR; Before Catalyst

Posted on May 20, 2009

  • 2359 Answers

SOURCE: i need a wiring diagram for an o2 sensor on a

See page two on this PDF file HERE

Posted on Mar 06, 2010

  • 4977 Answers

SOURCE: I have my check engine

bank 2 is the bank that doesn`t have cylinder #1 in it.sensor 2 is the sensor after the catalytic or at the rear of the catalytic converter.with mazda use a good quality sensor(preferedly factory) as if its poor quality sensor it will set a code and cause also usually have to cut and splice the wires to the original plug to use them.try buying one from a parts store that you don`t have to cut and splice that way it theres a problem with the sensor you can get it replaced easier.

Posted on Apr 27, 2011

Testimonial: "awesome, but still a bit thanx and"

Add Your Answer

Uploading: 0%


Complete. Click "Add" to insert your video. Add



Related Questions:

2 Answers

What means catalyst Efficiency below normal

      Got it scanned and it's P0420: Catalyst efficiency below thresholdbank 1. They said it could be a bad catalytic converter or oxygen sensor. I borrowed a scanner and tried to measure the oxygen sensor readings over time.Sep 8, 2011

      civic - Check Engine code P0420 Catalyst efficiency below ...

    P0420 Error Code: Catalyst System Efficiency Below ...
    P0420 OBD-II Trouble Code. Technical Description. Catalyst System Efficiency Below Threshold (Bank 1). What does that mean? This diagnostic trouble code ...

    P0420 Code Fix How Its Done Catalyst Efficiency Below ...

    ? 4:25
    Jun 20, 2014 - Uploaded by ACE AUTOS UK
    Here is a tutorial on P0420 code fix catalyst efficiency below threshold How To Diagnose a p0420 code ...

Jan 26, 2016 | Cars & Trucks

1 Answer

Code p0420 acura m'dx

1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Possible Causes: Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
Diagnostic Help:
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101", if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.

Jan 20, 2014 | 2005 Acura MDX

1 Answer

P0420 Problem Codes

Catalyst efficiency below threshold. It means your catatlytic converter is not working effectively. Make sure you have no exhaust leaks. If you have access to a scanner that can read live data look at the rear O2 sensor and compare the data to the front O2 at steady cruise. The front O2 will change constantly from 0 to 1 (.010-.089) volt once it starts to work. The rear O2 should change much it should stay in the mid range with very little fluctuation. If it does fluctuate a lot and you have eliminated O2 failure, vacuum and exhaust leaks then you have a bad catalyst which will require replacement.

Jun 06, 2012 | 2004 Toyota Sienna

1 Answer

2001 Toyota Camry - check engine light on - code p0420 - took to mechanic and he just hooked up code scanner and gave p0420 code - without checking anything else he said rear O2 sensor needs to be...

Hello: Code 420 normally indicates a catalytic converter is operating below 60% efficiency. You need to take your car to another shop and request a COMPLETE diagnosis not a dart board guess.

Sep 13, 2011 | 2001 Toyota Camry

1 Answer

Have a p0420 code on 2000 celica should i replace o2 sensors or cat

The catalytic convertermag-glass_10x10.gif has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Feb 17, 2011 | 2000 Toyota Celica

2 Answers

Check engine light is on. Code reads p0420? Is it a sensor or something else?

HI. This is a sign of a catalytic converter that is not working as efficiently as it should be. It is part of the vehicle emissions system. The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream O2 sensor waveform readings should fluctuate, but the downstream O2 sensor reading should be fairly steady. If the PCM detects that the two O2 sensors are reading in a similar manner, the DTC code P0420 will be issued. The necessary action are to check for any exhaust leaks at the manifold, pipes and catalytic converter. You will also need to check the spark timing to insure that it is normal and inspect the coolant temp sensor for its integrity. If all the check points test out ok, the remaining actions will be to replace your catalytic converter. A lot of the emission parts are covered under the warranty package if the vehicle is not out of its warranty contract. If you still have full warranty coverage at this time, this repair should be covered. I would check that out before purchasing any parts.

  • NOTE_ This code could also be triggered by using a leaded fuel in the vehicle. Make sure you are using the correct fuel blend. I should also point out that this is not a sensor issue. Your sensor(s) are actually functioning as designed. If they were not, you would not have this code. With that said; it wouldn't hurt to test them, if possible.

Aug 24, 2010 | 2002 Nissan Altima

1 Answer

Which is what I do to write the code P0420, as do I fix it? I change it?

So what is causing my P0420 code? There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc.
OK, so I want to diagnose things, what do I do next? The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those syptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.
On some vehicles such as some Subaru models an ECM reflash will fix the problem. As you can see a P0420 can be caused by many things, so if you have any doubts as to whether you can fix the problem yourself, please seek the advice of a professional technician. Good luck!

Answer found @

Jan 23, 2010 | 2002 Jaguar X-Type

1 Answer

I have a P0420 code on a nissan help me fix it

catalytic converter efficiency below threshold bank 1 possible problems are engine coolant temp sensor or o2 sensor not reading properly or front and behind catalytic o2 sensors read similar-- ****** spark timing--damaged or leaking /exhaust manifold/catalytic conv/exhaust pipe. check for leaks or damage to exhaust repair or replace as necessary

Nov 15, 2009 | 2005 Nissan Altima

3 Answers

Pulled code p0420 on 99 chevy astro, 2wd, v6

from what i am finding the code means o2 sensors. they recommend replacing them and if this does not work than replacing the cat converter

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.


A code P0420 may mean that one or more of the following has happened:
  • Leaded fuel was used where unleaded was called for
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
  • Retarded spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of readings
possible soloutions

Some suggested steps for troubleshooting a P0420 code include:
  • Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2), replace if necessary
  • Replace the catalytic converter
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

thank you for choosing fixya and please do not forget to comment and rate on your experience.

Aug 20, 2009 | 1999 Chevrolet Astro

Not finding what you are looking for?
2004 Mazda 3 Logo

437 people viewed this question

Ask a Question

Usually answered in minutes!

Top Mazda Experts


Level 3 Expert

80339 Answers

Colin Stickland
Colin Stickland

Level 3 Expert

22326 Answers

Jeffrey Turcotte
Jeffrey Turcotte

Level 3 Expert

8841 Answers

Are you a Mazda Expert? Answer questions, earn points and help others

Answer questions

Manuals & User Guides