P0420 GMC code: Catalyst system efficiency below threshold

P0420 GMC Code

This page will describe the symptoms. GMC P0420 trouble code. This article covers GMC P0420 symptoms, causes and potential solutions. GMC OBDII P0420 can often be caused by a defective oxygen sensor. However, there are other possible causes.

The most commonly encountered trouble code for GMC vehicles, the P0420. This is the code that you get when you connect your GMC to an OBDII scaner. It doesn’t matter which model you have, this GMC OBDII code has the same meaning.


P0420: Catalyst System Efficiency below Threshold


Índice de Contenido
  1. GMC P0420 OBDII code Defined
  2. GMC Code P0420 Symptoms
  3. Here are the Top 5 Reasons P0420 is Found in a GMC
  4. GMC Code P0420 Possible Solutions

GMC P0420 OBDII code Defined

GMC P0420 Code Symptoms

P0420 This is 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 GMC, one just before the converter and another after it. One of their many duties is to determine the level of emissions coming into and out the converter.

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

GMC Code P0420 Symptoms

GMC P0420 OBDII Trouble Code

A malfunctioning oxygen sensor can cause the P0420 code to appear in your GMC. In most cases, the code will trigger the oxygen sensor that is upstream of 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.

  • 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 GMC P0420 Code.


Here are the Top 5 Reasons P0420 is Found in a GMC

OBD2 Code P0420 GMC

Many things can lead to the trouble code P0420 in your GMC. These are some of the more common.

  • 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 GMC’s exhaust. They can get clogged over time. Even though modern catalytic convertors are designed to last most vehicle's lifetime, it is possible that it has become clogged.
  • O2 Sensor The wiring– Over time, the oxygen sensor wiring can go bad. Because it's right beside the exhaust, this wiring is extremely volatile. It is more distant from the ECM which makes it even more volatile.
  • Spark Timing– If your GMC 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.
  • Motor Temp Sensor– If the computer doesn’t know what the engine temp is it’ll keep the fuel mixture rich. The O2 sensor could be thrown if the mixture of fuel and oil is too rich.


GMC Code P0420 Possible Solutions

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. An exhaust leak should sound easily beneath the car.

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.


You will be able to determine the cause of P0420 in your GMC. Please let us know if you find any errors or suggestions to improve this article. Many thanks.

¡Más Contenido!

Leave a Reply

Your email address will not be published. Required fields are marked *

Go up