P0420 code Chevy: Catalyst system efficiency below threshold

Chevy P0420 Code

The most frequently encountered trouble code for Chevy vehicles Is the Code P0420. The code refers to a number you receive when your Chevrolet is plugged into an OBDII scanner.  It doesn’t matter which model you have, the P0420 Chevy OBDII code has the same meaning.


P0420: Catalyst System Efficiency below Threshold


Índice de Contenido
  1. Chevrolet P0420 OBDII Code Defined
  2. Chevy P0420 Code Symptoms
  3. The Top Reasons for P0420 in Chevy
  4. Possible Solutions for the Chevy P0420 code

Chevrolet P0420 OBDII Code Defined

Chevrolet vehicle P0420 Code Symptoms

P0420 An OBDII trouble code. Technically, the code stands for:

The Catalyst Systems Efficiency is Below the Threshold

At least two oxygen sensors are installed in your Chevrolet, one just before the catalytic convertor and another after.  One of their many duties is to determine the levels of emissions coming into and out the converter.

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

Your vehicle's software believes the catalytic convert is not doing its job.

If your vehicle has the P0420 code, it won’t pass emissions so please be aware of that.

Chevy P0420 Code Symptoms

Chevrolet P0420 OBDII Trouble Code

The P0420 code in a Chevrolet will often be thrown one of your oxygen sensors are not functioning properly. In most cases, the code is triggered from the oxygen sensor above 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.

  • Mileage- 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 Chevy P0420 code.


The Top Reasons for P0420 in Chevy

OBD2 Code P0420 Chevrolet

Many things can lead to the P0420 code on your Chevy.  Below are the top ones.

  • Oxygen Sensor– The P0420 codes is thrown when the oxygen sensors on both sides of the exhaust have very similar readings. 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).
  • Catalytic Converter– A catalytic converter is responsible for scrubbing out as much pollution as possible from the Chevy’s exhaust.  They can get clogged over time.  Even though modern catalytic convertors are designed to last for most vehicles' lifetimes, it is possible that there may be an underlying issue if they become clogged.
  • O2 Sensor Connectivity–  Over time, the oxygen sensor wiring can go bad.  Because it's right beside the exhaust, this wiring is extremely volatile.  Because it is further from the ECM, it can be volatile.
  • Spark Timing–  If your Chevy is misfiring or the exhaust timing is off, this can affect the gasses that are actually going to the O2 sensors enough to cause the P0420 code to register.
  • Exhaust Leak– 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 cause the O2 sensor to see an exhaust that is out of normal range. This could result in the sensor being thrown.

Possible Solutions for the Chevy P0420 code

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.  These are some great guides that will help you.


You will be able to identify the cause of P0420 in your Chevrolet. You can comment on the article or send a message if there are inaccuracies or any suggestions. Many thanks.

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