Acura MDX P0420: Catalyst System Efficiency → Below Threshold → Bank 1


Acura MDX P0420 Diagnosis


This page will help you to troubleshoot your problem. Acura MDX - P0420 trouble code. The P0420 definition, symptoms, causes, as well as possible solutions is covered. OBDII P0420 often occurs due to a bad oxygen sensorThere are many other reasons. You could waste your time and expense by rushing to replace the O2 sensors.  The most commonly used OBDII code is P0420.


Índice de Contenido
  1. Acura MDX OBDII Code Definition
  2. Acura MDX P0420 Symptoms
  3. Top Causes and Solutions of P0420 In Acura MDX
    1. Oxygen Sensor
    2. Catalytic Converter
    3. The O2 Sensor Wiring
    4. Timing
    5. A Leakage in the Exhaust
    6. Engine Temperature Sensor
  4. Conclusion

Acura MDX OBDII Code Definition

P0420 This is an OBDII Trouble Code. This code is technically:

The Catalyst Systems Efficiency is Below the Threshold

There are at least two oxygen sensors on your MDX. One is before the catalytic converter, and one after. One of their many duties is to determine the levels of emissions coming into and out the converter.

The vehicle will light the service engine when the oxygen sensor readings show identical readings (P0420).

These readings shouldn’t match, since the catalytic converter is supposed to be scrubbing the exhaust. The MDX software will detect a problem and notify you.


Acura MDX P0420 Symptoms

P0420 Acura MDX

Your oxygen sensors may not be working correctly and the Acura MDX's P0420 will display. The code is usually triggered by an oxygen sensor upstream (before the converter).

There aren’t usually any drivability issues associated with P0420. The first indication that something is amiss is usually the soon-light coming on in the service engine.

These are some of the symptoms you should look out for if your code is P0420.

  • Fuel Efficiency- It is possible for your catalytic converter to become clogged.
  • The loss of power– It is a really strange feeling to drive a vehicle with a catalytic converter that is going out. Typically, they’ll idle ok and drive normal under a light load. Whenever the engine is put under a heavy load it’ll feel like it is running out of gas. It’s very similar to the symptoms of a bad fuel filter.
  • Service Engine Light– Often, this is the only symptom of the Code Acura MDX code P0420.

Similar to: P0442 Acura MDX


Top Causes and Solutions of P0420 In Acura MDX

Your MDX can be affected by many factors, including the P0420 code. Below are the top ones.

P0420 can often be caused by an oxygen sensor. Specifically it’ll usually be the O2 sensor(s) on the upstream side of the catalytic converter.  But, don’t be surprised if it’s not. Checking the exhaust for signs of leakage is a smart idea. It should be possible to detect an exhaust leak underneath the vehicle.

Unless you have an exhaust leak, you’ll probably need to test the O2 sensors and/or catalytic converters. These are some great guides that will help you.


Oxygen Sensor

When the readings of both the exhaust oxygen sensors have similar readings, the P0420 code is thrown. An oxygen sensor that is defective can trigger the code by giving a false reading. If the engine does not exhibit any of the decreased performance associated with a bad catalytic converter, it very well may be the oxygen sensor (if you haven’t noticed the performance loss yet).

How to Test an O2 and Oxygen Sensor (YouTube)

New Oxygen Sensor – Acura MDX (Amazon)


Catalytic Converter

A catalytic converter is responsible for scrubbing out as much pollution as possible from the MDX’s exhaust. These can become blocked over time. Even though modern catalytic convertors are designed to last for most vehicles' lifetimes, it is possible that they have become blocked.

You can test your catalytic converter in just 15 minutes (YouTube)


The O2 Sensor Wiring

Oxygen sensor wiring can become damaged over time. Because it's right beside the exhaust, oxygen sensor wiring can become extremely volatile. It is volatile because it is further from the ECM.

In less than an hour, test your wiring (2 Auto Pros)



The P0420 registration code can be caused by a misfiring MDX or an incorrect exhaust timing.

A Leakage in the Exhaust

If there’s a noticeable exhaust leak coming from the vehicle it can change what the O2 sensors register enough to throw the P0420 code.

Engine Temperature Sensor

If the computer doesn’t know what the engine temp is it’ll keep the fuel mixture rich. If the fuel mixture becomes too rich, it will not see the exhaust that the O2 sensors can detect and could throw the sensor.



You will be able to determine the cause of P0420 in your Acura MDX. Please feel free to leave a comment or message if you have any suggestions. Many thanks.

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