P0420 Code (Symptoms, Causes, and Fixes)

There are affiliate links on this article. If you make a purchase through any of the links, I may earn a small commission at no extra cost to you.

One of the fastest ways to detect what’s wrong with your car is by troubleshooting the central computer using an OBDII scanner or any other auto scanner tool.

But, when you use auto-scanning tools, the responses appear as DTCs (Diagnostic Trouble Codes), and as such, if you don’t understand DTCs, it’d still be difficult to figure out what was wrong with your car.

The P0420 code is a DTC that indicates your car’s catalyst converter is experiencing an issue. This error needs to be fixed as early as possible because the catalyst system in modern vehicles plays an important role.

What Does the P0420 Code Mean?

Technically, this code means: “Catalyst System Efficiency Below Threshold (Bank 1).” Just as it reads, the error code tells that your car’s catalyst converter is not performing as expected.

Note: “Bank 1” refers to the side of the engine that houses the number one cylinder. Its opposite is “Bank 2.”

The catalyst converter (shortened as CAT) is responsible for breaking down the harmful emissions from the engine – turning them into less-harmful gases, which are passed through the exhaust’s tailpipe onto the atmosphere.

There are sensors installed around the catalyst convert to monitor its performance. These sensors send signals to the ECU/ECM, which processes the signals to ascertain if your car is running as support.

If the ECM receives signals that the CAT isn’t functioning as expected, it’d trigger the check engine light to come, and you may experience pretty other unusual signs.

However, it is important to note that in some cases, the CAT may still be doing its job perfectly, but the Oxygen (O2) sensors installed around it are faulty, which is why the ECM is receiving wrong signals that cause the check engine light to illuminate and also possibly cause the P0420 code.

How Serious is the P0420 Code?

This is a serious issue that should be fixed as early as possible. A bad catalyst converter can affect the overall performance of an engine, and in the long run, can cause the engine to break down.

Symptoms of P0420 Code

Listed below are the commonest symptoms you’d most likely experience if your car is going to throw a P0420 code when you troubleshot.

1. Engine Misfires

You’d experience constant misfiring when your car is facing CAT-related issues. This happens because the catalyst converter plays an important role in the combustion process.

2. Check Engine Light is Illuminated

Definitely, your car computer would illuminate the check engine light/icon once it receives wrong signals from the O2 sensors installed in the front and back of your vehicle’s CAT.

3. Power Loss / Poor Acceleration

If you always notice that your engine losses power frequently – unlike ever, that could be a sign of a failing catalyst converter. Also, a bad CAT can affect your car’s acceleration, making it impossible for you to go beyond the 30 – 40mph range.

4. Rotten Egg Smell

A bad catalyst system won’t effectively break down the unburnt gas and emissions from the combustion chamber, which can cause you to always perceive rotten egg smell coming from the exhaust tailpipe.

Causes of P0420

There are pretty many reasons why your car would throw a P0420 DTC when you troubleshot. Here is a list of the most common causes:

How to Fix the P0420 Code

Symptoms of P0420 Code

Fixing this code can be quite expensive if it implies getting a new catalyst converter. However, if the fault was from the O2 sensors, that’s a pretty cheaper fix. Notwithstanding, here are some tips on how to get around this error and clear it from your car’s computer.

1. Clean/Unclog The Catalyst Converter

Start with cleaning the CAT; check if it’s clogged and remove the clog. Also, you can get a catalyst converter cleaner and clean it out.

You may also need to clean other components connected to the converter such as the muffler and exhaust manifold. This simple act can get things back to normal.

2. Replace Faulty O2 Sensors

Troubleshoot the car computer to find out if the O2 sensors are faulty. If the upstream (front) or downstream (back) oxygen sensor is faulty, it can cause the P0420 code to appear.

3. Inspect for Leaks and Fix Them All

Carefully check for leaks on the catalyst converter and other components that are installed closely. If you find any leak, replace the component or find a way to cover the leaking spot.

4. Change the Air Fuel Sensor

Sensors might be tiny, but they play crucial roles in the effective running of an automobile’s engine. A faulty air-fuel sensor can trigger the P0420 error code, so you should have the sensor checked for failure and have it fixed.

Cost of Fixing the P0420 Code?

The cost depends on what was fixed; installing a new CAT can cost from $500 – $2,500 (plus labor cost). New O2 sensors cost around $200 and welding a leaky spot can cost $100.


This article has clearly explained the P0420 DTC, which indicates a problem with your car’s catalyst converter.

After fixing the faulty component that caused the code to appear, you need to clear the code using the OBDII scanner. Fixing this error code is quite technical, so it’s best to engage a mechanic.

Scroll to Top