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


Acura TL P0420 Diagnosis


This page will help you to troubleshoot your problem. Acura TL - P0420 trouble code. The P0420 definition, symptoms, causes, as well as possible solutions is covered. OBDII P0420, often caused by an ailment like a TL or 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. Definition of Acura TL OBDII code P0420
  2. Acura TL Code P0420 Symptoms
  3. Top Causes and Solutions to P0420 In Acura TL
    1. Oxygen Sensor
    2. Catalytic Converter
    3. Connecting O2 Sensors
    4. Timing
    5. A Leakage in the Exhaust
    6. Engine Temperature Sensor
  4. Conclusion

Definition of Acura TL OBDII code P0420

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

The Catalyst Systems Efficiency is Below the Threshold

At least two oxygen sensors are installed in your TL, one just before the converter and another after it. The oxygen sensors are responsible for measuring the levels of the converter's emissions.

If the downstream and upstream oxygen sensors give similar readings, then the vehicle will soon turn on the service engine (P0420).

These readings shouldn’t match, since the catalytic converter is supposed to be scrubbing the exhaust. The software on your TL will determine if the numbers match.


Acura TL Code P0420 Symptoms

P0420 Acura TL

If your oxygen sensor is not working properly, the P0420 code will be displayed in Acura TL. 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.

If you are assigned code P0420, these are common symptoms.

  • Fuel Efficiency- You may or not experience any problems depending on the extent of catalytic conversion blockage.
  • Perilous times for 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 Acura TL code P0420.

Similar Articles: P0442 Acura LTL


Top Causes and Solutions to P0420 In Acura TL

You can have the trouble code P0420 in your TL for many reasons. Below are some common causes.

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. You should check your exhaust first for any leaks. 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. Below are some good guidelines to assist you in this task.


Oxygen Sensor

If the two exhaust oxygen sensors have identical readings, then the code P0420 is thrown. If the oxygen sensor fails, the code can be triggered. 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 or Oxygen sensor (YouTube)

New Oxygen Sensor – Acura TL (Amazon)


Catalytic Converter

A catalytic converter is responsible for scrubbing out as much pollution as possible from the TL’s exhaust. These can build up over time. Even though modern catalytic convertors are designed to last the lifespan of vehicles, there may be an issue with their functioning.

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


Connecting O2 Sensors

The oxygen sensor wiring may become corroded over time. Because it's right beside the exhaust, oxygen sensor wiring can become extremely volatile. Because it is far away from the ECM, the downstream oxygen sensor wiring can also be volatile. (More distance = greater chance of failure).

You can test wiring in under an hour (2 Auto Pros)



The P0420 registration code can be caused by a misfiring TL 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.



It's your task to find the root cause of Acura TL P0420. Please feel free to leave a comment or to send a message if you have any suggestions. Many thanks.

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