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P0421 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


Inside your car is a catalytic converter, which reduces the emissions and toxins inside your exhaust gas. Each vehicle manufacturer has recommendations regarding the efficiency of this component when warming up the car. If your powertrain control module (PCM) detects that the converter is operating at a lower rating, it will trigger the P0421 code.


The two sides of the engine are referred to as banks. Bank one is the front end, while bank two is the rear. Code P0421 means that there is an error in bank one. Typically, this code will show up between five and ten minutes after starting the car since the O2 sensor needs time to adjust.


Fortunately, this code most often relates to the efficiency of your car’s catalytic converter. In almost all cases, the converter is faulty, and either needs to be repaired or replaced. However, if there are additional PCM codes stored when checking your scanner, the cause could be somewhere else in the engine. Other potential triggers for this code can include:

  • Faulty Oxygen Sensor – this sensor is calibrated to monitor the efficiency of your converter. If it’s broken, it could give an incorrect reading.
  • Damaged O2 Sensor Circuitry – while the sensor itself may be faulty, the problem could lie in the wiring connecting the sensor to your engine.
  • Faulty Powertrain Control Module – in some cases, your PCM may not be working correctly, which is why it’s triggering codes for problems that don’t exist.


Your catalytic converter can impact your vehicle’s performance substantially. If you notice any of these warning signs, it’s time to get your engine checked.

  • Check Engine Light Comes On – be sure to use your OBD2 scanner to see if there are additional codes that need to be addressed.
  • Engine Doesn’t Start – if the converter is busted, you may not be able to get the engine to turn over at all.
  • Lack of Power or Acceleration – when your gas isn’t burning efficiently, it can cause the car to feel sluggish. You may also notice the vehicle drag when pressing the gas pedal.
  • Strange Noises While Driving – unfortunately, noises coming from the engine can mean a variety of problems. However, a broken or faulty catalytic converter will usually start to rattle, and the sound can get worse the longer you drive.

Also Read: Why Does My Car Make Noise When Turning


As we mentioned, if code P0421 is the only one stored in your PCM, then the converter is likely to blame. Also, since the code specifies bank one, you can start by inspecting there. You’ll want to look for signs of damage, as well as smell the converter. When this component breaks down, or there’s excessive fuel in the system, it will have a pungent sulfur odor to it.

To further test the converter, you can inspect it while the engine is running. If it starts rattling or turns red, then it needs to be replaced.

If a visual inspection doesn’t yield any results, then you will need to check the oxygen sensor and its wiring. If the sensor works fine, then the problem is likely the PCM itself.

Common mistakes

Since a broken catalytic converter often causes this code, it’s easy to assume that changing it will fix the problem. However, be sure to go through the full inspection so that you can rule out any other potential causes. Also, if there are other codes stored in the PCM, the converter may only be one of several issues that need correcting. Don’t ignore additional codes when checking the engine. 

How serious is this?

Think of the catalytic converter as the lungs of your car. If they are damaged or clogged, your engine can’t breathe. Imagine trying to run a marathon with a collapsed or infected lung. This code is severe and should be addressed immediately. Driving with a faulty converter will only get worse, and it could lead to catastrophic damage to the engine itself.

It’s also crucial to note that you have to diagnose and repair this problem as soon as you notice symptoms. If the car is sluggish during acceleration, that means the converter is starting to fail. Waiting will almost certainly damage it further, which can lead to more costly repairs.

What repairs can fix the code?

Replacing your catalytic converter should take care of the problem. However, if the cause was a faulty wire or sensor, then replacing those elements will also clear the code.

Whenever you repair or replace something on the engine, be sure to clear the code before driving. If the check engine light comes on again, then it means you will have to run diagnostics and inspect each component again.

Related codes

As we mentioned, there are two banks on your engine. Code P0431 refers to bank two.


You can’t afford to wait to repair or replace your catalytic converter. If you need assistance in testing your sensors or the converter itself, be sure to take your vehicle in to a certified mechanic. The longer you drive with this problem, the more expensive it will be.

P0421 Code – What Does It Mean & How To Fix It