This page is meant to define the symptoms of the GMC P0420 trouble code. It covers GMC P0420 meaning, symptoms, causes, and possible solutions. GMC OBDII P0420 is often caused by a bad oxygen sensor, but there can be other causes as well.
One of the most common trouble codes with all GMC vehicles is the P0420. This code number references a number that is given to you when you plug your GMC into a OBDII scanner. It doesn’t matter which model you have, this GMC OBDII code has the same meaning.
P0420: Catalyst System Efficiency Below Threshold
GMC P0420 OBDII Code Defined
P0420 is a OBDII trouble code. The code technically stands for:
Catalyst System Efficiency Below Threshold
Your GMC has at least two oxygen sensors, one before and after the catalytic converter. Among the many jobs that oxygen sensors have is they have to measure the emissions levels coming in and out of the converter.
When the readings from the upstream and downstream oxygen sensors show similar readings, the vehicle will throw the service engine soon light (P0420).
The software in your vehicle believes that the catalytic converter is no longer doing its job.
If your vehicle has the P0420 code, it won’t pass emissions so please be aware of that.
GMC P0420 Code Symptoms
The P0420 code in a GMC will often be thrown one of your oxygen sensors are not functioning properly. Typically, the code will be triggered by the upstream oxygen sensor (before the converter).
There aren’t usually any drivability issues associated with P0420. For most people the first sign that anything is wrong at all is the service engine soon light coming on.
Here are the typical symptoms that something is wrong when you have code P0420.
- Mileage- May or may not suffer depending on how much the catalytic converter is clogged.
- 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.
Top Causes of P0420 in a GMC
There are many things that can cause the P0420 trouble code in your GMC. Here are the most common ones.
- Oxygen Sensor– The P0420 codes is thrown when the oxygen sensors on both sides of the exhaust have very similar readings. When the oxygen sensor is bad, it can cause a false reading and trigger the code. 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. Over time, they can become clogged. Although, modern catalytic converters are supposed to last the life of most vehicles, it could be there is some underlying problem if it has clogged.
- O2 Sensor Wiring– Over time, the oxygen sensor wiring can go bad. It is especially volatile since it is right next to the hot exhaust at all times. The downstream oxygen sensor wiring is especially volatile since it is so far from the ECM (more distance to travel.
- 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.
- Engine Temp Sensor– If the computer doesn’t know what the engine temp is it’ll keep the fuel mixture rich. When the fuel mixture is rich, the exhaust the O2 sensors see will be out of range and it could throw the sensor.
GMC P0420 Code Possible Solutions
Often, the oxygen sensor is the cause of P0420. 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. It would be a good idea to check the exhaust for leaks first. You should be able to hear an exhaust leak easily underneath of the vehicle.
Unless you have an exhaust leak, you’ll probably need to test the O2 sensors and/or catalytic converters. Here are a couple of good guides to help with that.
Good luck figuring out what is causing P0420 with your GMC. If there are any inaccuracies, or anything that you feel could improve the article, please feel free to comment or message. Thank you.