Saturn SW P0420: Catalyst System Efficiency → Below Threshold → Bank 1


Saturn SW P0420 Diagnosis


This page is meant to help you troubleshoot the Saturn SW P0420 trouble code. It covers P0420 meaning, symptoms, causes, and possible solutions. WIth the SW, OBDII P0420 is often caused by a bad oxygen sensor, but there can be other causes as well. Jumping straight to replacing the O2 sensor can cost you time and money. P0420 is the most common OBDII code there is.


Saturn SW P0420 OBDII Code Defined

P0420 is a OBDII trouble code. The code technically stands for:

Catalyst System Efficiency Below Threshold

Your SW 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).

These readings shouldn’t match, since the catalytic converter is supposed to be scrubbing the exhaust. When the numbers match, the software in your SW believes that the catalytic converter is no longer doing its job.


Saturn SW P0420 Code Symptoms

P0420 Saturn SW

The P0420 code in a Saturn SW 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.

  • Fuel Efficiency- 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 Saturn SW P0420 code.

Related: P0442 Saturn SW


Top Causes and Solutions of P0420 in the Saturn SW

There are many things that can cause the P0420 trouble code in your SW. Here are the most common ones.

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.


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 (or you haven’t noticed the performance loss yet).

How to Test an Oxygen or O2 Sensor (YouTube)

New Oxygen Sensor – Saturn SW (Amazon)


Catalytic Converter

A catalytic converter is responsible for scrubbing out as much pollution as possible from the SW’s exhaust. Over time, they can become clogged. Although, modern catalytic converters are supposed to last the life of most vehicles, os there could be some underlying problem if it has clogged.

How to Test Your Catalytic Converter in 15 minutes (YouTube)


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 also volatile since it is so far from the ECM (more distance to travel = more area to fail).

How to Test Wiring in Under an Hour (2 Car Pros)



If your SW 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.



Good luck figuring out what is causing P0420 with your Saturn SW. If there are is anything that you feel could improve the article, please feel free to comment or message. Thank you.