Need car parts? Select your vehicle
Reading Time: 6 minutes

What Does the P0421 Code Mean? 

Code P0421 indicates a problem with the warm-up catalytic converter (also known as the “Light Off Cat”). That’s the catalyst nearest the engine. There is another catalyst behind this catalyst – that’s not the one this code refers to. That rear catalyst isn’t monitored. This one is.

Note that a P0421 code is not listed on most vehicles. The most common vehicle that will store this code are Mazdas and some Mitsubishi vehicles.

car under chassis showing catalytic converter
Although the trouble code P0421 is related to your vehicle’s catalytic converter, it does not always mean that the converter is faulty. It could also indicate other issues, such as a circuit problem or a bad oxygen sensor.

For example, this is how a Mazda decides whether or not to store a P0421:

  • The PCM monitors the catalytic converter’s oxygen storage capacity.
  • The PCM temporarily changes the target air/fuel ratio to rich at “fuel cut” recovery, and detects the response time until the HO2S output signal outputs the voltage on the rich side.
  • If the catalytic converter purification characteristic deteriorates, the amount of the time until the HO2S detects the voltage on the rich side is shorter compared to the normal purification characteristic.
  • The PCM detects a catalytic converter malfunction using this performance characteristic.

Although the trouble code P0421 is related to your vehicle’s catalytic converter, it does not always mean that the converter is faulty. It could also indicate other issues, such as a circuit problem or a bad oxygen sensor

The main function of the oxygen sensor is to monitor your vehicle’s exhaust output and send the data to your vehicle’s powertrain control module (PCM). The PCM then compares the data from the upstream and downstream oxygen sensors to determine catalyst efficiency.

If you want to learn more about catalysts, the role they perform in a vehicle’s emission control system, and how OBD2 codes can be triggered by this system, read our technical explanation here.

To learn the possible causes of P0421, read the next section.

What are the Possible Causes of the P0421 Code? 

Here are some of the common triggers of the error code P0421: 

  • Faulty Bank 1 catalytic converter
  • Faulty oxygen sensor
  • Issues with the oxygen sensor circuit, such as damaged wires or poor connections 
  • Exhaust leak 
  • An issue with the PCM, such as software in need of an update 

What are the Common Symptoms of the P0421 Code? 

Although P0421 is a generic code, its symptoms may vary depending on your car’s year, make, and model. For example, the symptoms of the code P0421 for Mitsubishi vehicles may differ from error code P0421 for Mazda cars. Here are some of the most common symptoms related to the P0421 code: 

This is the most common warning sign of the P0421 code. As a general rule, it is recommended to have your vehicle checked by a mechanic once the check engine light illuminates, regardless of the trigger. 

  • Lack of engine power

In some cases, the catalytic converter may become clogged, creating excessive exhaust back pressure. This can result in a lack of engine power.  

How to Diagnose the P0421 Code

Since P0421 is a generic code, it can be seen in various makes and models. However, keep in mind that the diagnostic and repair steps may vary depending on your vehicle’s make and model. Here is a video to help you understand what the troubleshooting process might involve: 

How to Fix the P0421 Code

Although the P0421 code shares many causes, symptoms, and areas with numerous engine codes, this does not mean that it can be fixed with a generic solution. Hence, it is usually an issue that’s best left to the experts if you are not confident with your automotive repair skills.

The first step to resolving the P0421 code is diagnosis. Once you’ve determined what triggered the code, determine the appropriate fix with the help of online auto repair resources and guides.

You can also supplement your repair task with more information about the P0421 code while building up your repair know-how in case of future issues. Get an ALLDATA single-vehicle subscription for in-depth and up-to-date factory guides.

a catalytic converter of a modern car close up view
In some cases, a clogged catalytic converter, which can cause a decline in engine power, may be a symptom of code P0421.

A Lesson on Catalysts and How they Can Trigger OBD2 Codes

The catalytic converter is mounted in the exhaust system and basically looks like a muffler. All the exhaust passes through the catalytic converter, and engines with two banks will have more than one converter. When working properly, the converter chemically transforms pollutants created by the engine into harmless water vapor and carbon dioxide.

The three enemy gasses the emissions system targets are

  • NOx (Oxides of Nitrogen)
  • HC (Hydrocarbons, or unburned fuel)
  • CO (Carbon Monoxide, which is invisible, odorless, and deadly poison in a closed space).

A “catalyst” changes other things it comes in contact with without undergoing change itself. Today’s vehicles have what is known as a three-way catalyst that handles all three of the harmful gasses.

The catalytic converter has a hard clay “honeycomb” coated throughout with very fine particles of precious metals like platinum, palladium, rhodium, etc. Those precious metals are the chemical element of the catalytic converter, which must get very hot (over 1,000 degrees) before it begins to do its work. That’s why the “light off” (warm up) catalyst is mounted so near the engine.

As for the P0421 code, it’s important to note that it only points to a problem with the warm up converter that is mounted closest to the engine. That’s also true of the P0420 and P0430 codes. As stated earlier, the rear converter isn’t monitored by the OBD 2 system. Some converters have both “bricks” in one shell with the warm up cat’s special monitoring O2 sensor screwed right into the middle of the converter between the two bricks.

Catalysts and Emissions

The warm up converter nearest the engine handles NOx emissions (a product of high combustion temperatures) and the catalyst farthest from the engine handles HC and CO (from too-rich mixtures). A healthy engine with a healthy catalyst system will produce CO2 and water vapor at the exhaust, both of which are actually beneficial to plants and the planet. As you’re breathing right now, you’re producing water vapor and CO2 just like a healthy vehicle.

Remember, normal combustion creates water vapor – that’s why all that steam comes from the exhaust until the exhaust system warms up when you start a vehicle in cold weather. An engine typically produces a gallon of water for every gallon of fuel consumed. Is this a problem? Not at all. Neither is CO2. The grass and trees love CO2; as a matter of fact, they can’t survive without it, and as they consume it, they give off oxygen – which we need just as badly as they need CO2. That plant-feeding CO2 is one of the reasons they have to mow the roadsides so much.

Since 1996, OBD2 (On Board Diagnostics 2) regulations have required the PCM (that’s the vehicle’s engine computer) to keep track of the catalytic converter’s performance by monitoring the downstream oxygen sensor; that’s the one behind the warm up catalytic converter.

If you’re advanced enough to watch live data on a scan tool or with your smartphone app talking to a dongle connected to the DLC connector on a healthy engine, you’ll see the upstream O2 sensor the computer uses to balance the air/fuel mixture switching rapidly (several times per second) and you’ll see the downstream corresponding O2 sensor switching much slower.

That’s the way it’s supposed to be, you see. When the downstream sensor begins to switch at the same rate as the upstream sensor, the P0420/P0430 codes are stored by the PCM to flag the failure of the catalyst to store oxygen, which is something it’s supposed to do.

Products Mentioned in this Guide

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

article subscribearticle subscribe
Exterior Accessories
Notify of
Inline Feedbacks
View all comments

View all Questions & Answers Answers BE PART OF OUR COMMUNITY: Share your knowledge & help fellow drivers Join Now
Copyright ©2022, Inc. All Rights Reserved.