Catalytic converters are all about efficiency. They have to be. Otherwise, the whole point of having a bunch of expensive metal in your exhaust system doesn’t make much sense. As with most things car-related, catalytic converters can fail. When that happens, you’ll get a check engine light code on your dash. 

There are several codes that can pop up due to a cat failure, and P0421 is among the more common ones. Join us as we go over what causes this code, what symptoms to look out for, and how to potentially fix it. 

What is the P0421 Code, and What Does It Mean?

The full definition of this code is “Warm Up Catalyst Efficiency Below Threshold (Bank 1).” It means that the voltage of the upstream and downstream sensors is identical. This happens when the catalytic converter experiences a reduction in emissions efficiency.

What Causes the P0421 Code? 

The causes of this trouble code can vary. Most engine issues will lead to reduced catalytic converter performance. The catalytic converter itself rarely fails on its own. That being said, here are the most common causes of a P0421 code:

• Failed catalytic converter
• O2 sensor issues
• Wiring harness failure
• Spark plug contamination

Failed Catalytic Converter

The catalytic converter substrate is made of rare earth metals. Under normal operating circumstances, they are designed to last until the wheels fall off. However, when the engine runs rich or experiences contamination issues, you’ll end up with a cat that is either clogged or damaged. 

When the downstream O2 sensor detects that the catalytic converter is no longer performing, you will get the P0421 code.

O2 Sensor Issues

This is a false-positive situation. This means that your catalytic converter may not have failed, but either the upstream or downstream O2 sensor has. O2 sensor failure can happen when either one gets clogged up with particles from the exhaust stream and when they fail internally. In both cases, you will get the P0421 code.

Wiring Harness Failure

Wiring harness failure can affect all sensors. When your O2 sensor no longer receives the correct amount of current, sends incorrect voltage readings or the ground wire is shorted, it can mean that the wiring harness has failed.

Spark Plug Contamination

This is one of the primary engine issues that can cause a catalytic converter to fail. Many problems cause the engine to run rich. Spark plug contamination is a telltale sign that something else is wrong with your engine. Additionally, they can fail, causing cylinder misfires.

What Is a Warm-Up Catalyst?

The warm-up catalyst solves the problem of emissions on a cold start. Essentially, the efficiency of a catalytic converter is optimal at a certain operating temperature. The warm-up catalyst helps mitigate emissions before the actual catalyst reaches operating temperature.

What Are the Common Symptoms of P0421 Fault Code?

Knowing how to identify the symptoms of a car issue not only helps prevent further damage but is also a vital part of the diagnostic process. Here are some of the common symptoms of the P0421 code.

• Check engine light
• Reduced performance 
• Rotten egg (sulfur) smell

Check Engine Light

The check engine light is your first indicator of a P0421. Of course, the CEL comes on for a whole range of issues. That being said, there is a reason why you’ve got a dedicated “something is wrong with the car” light. It’s there to let you know that driving the car might cause more damage. This is why it’s absolutely imperative that you get your car scanned as soon as possible when you see a CEL on your dash. 

Reduced Performance 

This is a common sign of catalytic converter issues. When the catalytic converter is clogged, the exhaust gasses can’t escape. This causes a back pressure in the upper part of the exhaust and power issues.

Rotten Egg Smell

Among the chemicals that your catalytic converter neutralizes, hydrogen sulfide is one of the easier ones to spot. This chemical gives off a distinct rotten egg smell. The CC converts it to sulfur dioxide, which has no smell. So, when you have a P0421, you might notice the smell of rotten eggs coming from the exhaust.

How to Diagnose and Fix the Cause of P0421 Code 

Now that you know the causes and symptoms, it’s time to diagnose the root cause. There are conclusive diagnostic methods to diagnose the P0421 code, like any trouble code. 

OBD-II Reader

To interface with your car’s engine computer, you need an OBD-II reader. This will not only help you identify the P0421 trouble code but will also help further diagnose the issue using live data and additional trouble codes. Run a full scan of the car to get all the codes. This is essential since diagnosing one code is a whole different ball game to diagnosing a set of codes. 

Visual Inspection

Although you can’t really take a look inside a catalytic converter, you can look for signs of corrosion, fraying, or exposed wires on the wiring harness and O2 sensor connector.

Although it is rare, there might be external signs of damage on the catalytic converter as well. Look for any holes on or near the cat, exhaust, and exhaust manifold. Any rattling coming from the area is also a cause for concern.

Multimeter Test

Ruling out external damage is only the first step in diagnosing a P0421 code. With the help of a multimeter, you will be able to conclusively diagnose whether the wiring harness or sensor has failed.

First, we need to test whether the wiring harness is functioning. Your wiring harness uses 3 wires, live (red), ground (black), and signal (blue). The colors differ between manufacturers, so it’s always good to consult your owner’s manual for the specific configuration.

You need to back probe the sensor connector, so you know the voltage is traveling to and from the sensor. Contact your black multimeter probe with the vehicle chassis. The red probe should contact the wire that you are testing. 

Your signal wire should give you around 0.1V when the vehicle is on but not running. When the vehicle is running, it should be between 0.1V and 0.9V. Your live wire should give you around 12V and your ground wire should be 0V. Additionally, the voltage on the upstream and downstream O2 sensors should not be the same. 

If the sensors are operating in the correct range but are identical, it means that there is an issue with the catalytic converter.

Vacuum Test

Often, the P0421 code can be caused by air leaks either in the engine or exhaust. If you’ve inspected your vehicle and found no prominent leaks, it is still possible that there are small leaks somewhere in your system. These leaks are notoriously difficult to find and a major reason why many owners abandon their vehicles when they have vacuum issues.

Attach your vacuum sensor to the intake hose near the intake manifold. When running your vehicle, you should get between 14HG and 22HG. This does not rule out leaks after the catalytic converter, but now we know that there are none before it.

Catalytic Converter Test

This test is a non-invasive way of testing your catalytic converter. Using a borescope is also a great option, but will set you back a bit more. 

You will need an infrared thermometer to test the temperature of the warm-up catalyst. First, let your vehicle run for at least 10 minutes. This is to allow the catalytic converter to reach operating temperature. 

The specific temperatures matter little here. At the top of your catalytic converter (inlet side), the temperature should be about a 100 °F less than at the bottom (exhaust side). If it is less than the temperature at the top it either means that your catalytic converter is clogged or is not functioning.

Another great test is to test for back pressure from the catalytic converter. You can either use a normal or exhaust back pressure gauge. Connect your pressure gauge to a point before the catalytic converter. On idle, you should get around 1.3PSI. When giving it some throttle (2500rpm), around 2.9PSI.

How to Fix a P0421

Unfortunately, fixing the P0421 code almost always implies replacing parts. Either the catalytic converter, or the O2 sensors will most likely have to be swapped out. The latter is definitely the preferable option since an O2 sensor is far cheaper than a brand-new cat. Speaking of which, you need to be absolutely sure that your catalytic converter has failed before you decide to pull the trigger on a new one. 

Get under the car and give the cat a good shake. If you hear any rattling noise coming from the inside, that’s a clear sign that your rare metal honeycomb structure is broken up and loose inside.

Does P0421 Clear Itself?

Catalytic converter codes are finicky. It is better to clear a P0421 code with an OBD-II reader. If that is not possible, it will clear within a drive cycle. The length of a drive cycle depends on the manufacturer, but it is by and large when the vehicle reaches operating temperature.

Is It OK to Drive with P0421?

Excessive back pressure can damage your engine. A short drive will not damage your vehicle. However, you should not drive your vehicle over an extended period with this trouble code.

Fix the P0421 Code With Quality Parts! 

Emissions trouble codes, like the P0421, can be painfully tedious. You now know almost all there is to know about the problem, but fixing it might take some new parts. 

We at eEuroparts.com have all the parts you need to fix the P0421 trouble code and more. Input your car’s data and then select the parts that you need from a list that is tailored for your vehicle.