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P0420 and P0421: Catalyst Trouble Codes

Whether you’ve noticed some performance issues or only receive an OBD-II scanner error code, you’re probably wondering, what exactly is the deal with trouble codes PO420 and P0421? These error codes are general indications that something is causing your catalytic converter to not function properly. However, getting to the bottom of exactly what’s causing that can be a bit of mechanical detective work. Here are some helpful steps to take to remove the pesky code.

  • P0420: Catalyst System Efficiency Below Threshold
  • P0421: Warm Up Catalyst Efficiency Below Threshold

What Does It Mean?

Your vehicle is equipped with at least one oxygen sensor, and if your vehicle is newer than 1995, multiple oxygen sensors that monitor the output of your exhaust system. This sensor is designed to monitor oxygen in the exhaust, in order to tell your engine’s control computer exactly what the air/fuel ratio is, so the computer can control the items that adjust that ratio. On all cars made after 1995, there are 2 types of oxygen sensors, what is referred to as an “upstream” and “downstream” sensor. The upstream sensor does most of the work in determining the air/fuel ratio of the vehicle, while the downstream sensor is located after your catalytic converter, and it’s there to monitor exactly how your converter is performing – or simply put, is it doing it’s job. When your downstream O2 sensor detects that the converter is not operating up to proper spec – it sends a warning. This warning comes in an engine trouble code – PO420 or PO421. Depending on the symptoms, there are a few likely causes and solutions.

Common P0420/P0421 Symptoms

For many drivers, the first sign that the catalyst efficiency is below the threshold is that the Check Engine Light turns on. In many cases, drivability will appear quite normal. Keep in mind that your car’s engine management system is very fine tuned to reporting problems with components within the system, so if other trouble codes appear at the same time, this is a good indicator of what is causing the problem. Other items that could be causing multiple trouble codes to appear with the 420/421 codes are:

  • Bad Oxygen Sensors (either upstream or down)
  • Engine Rich / Lean condition
  • Bad MAP or MAF sensor
  • Engine Misfire Codes

Each of these codes are very well likely indicators of what caused the 420/421 code to come on. It’s important to note that any combination of codes with the 420/421 code should be checked first as they are likely causing it. Where things can get really tricky, however, is when just the 420/421 code arrives by itself. It’s important here to remember, if another component, like an O2 sensor is bad, then you are going to receive a code for that as well. If there is no code for another component, then it’s almost certain that component is not at fault. A common misconception is that a 420/421 code means “your O2 sensor is bad”. If the O2 sensor was bad, you would be receiving a code telling you that.

Potential Causes of P0421

There are a number of reasons you may be seeing these error codes, but often times, the first place is to gravitate to the Catalytic Converter itself. Depending on the make and model of your vehicle, your catalytic converter should last at least 10 years, but numerous issues could reduce the lifespan. Clogging, physical damage, such as engine misfires or coolant and oil contamination can dramatically reduce the lifespan of your converter. It’s important to note here, that generally a Converter will not fail on its own. Something causes it to do so. Common issues that could be causing the converter’s output to be triggering the code could be:

  • Head gasket leak – leaking coolant into the exhaust
  • Excessive oil blow-by or oil consumption getting into the exhaust
  • Bad Gas – particularly high water content
  • Exhaust leak

Minor Solutions

Because your downstream oxygen sensor is the primary source of your P0420/421 error code, your first step should be to check for error codes around this sensor. If you choose to replace the sensor as routine maintenance, that’s fine, but normally if a code wasn’t present for the sensor, it’s not going to fix the 420/421 code. Check for bad exhaust leaks – even though normally you will clearly hear them. Any rattling coming from the catalytic converter is too things – either the heat shield surrounding the converter is rattingly, or it’s a good indicator that the converter’s inside element has been broken, damaged, and not flowing properly.

Make sure to check the electrical connectors leading to the downstream Oxygen Sensor, but remember again, if there is a fault here, it’s going to throw another trouble code.

Major Repairs

A damaged catalytic converter requires more intensive repairs, and while replacing the converter is the start, you must also find out exactly what caused it to fail. Many times, the exhaust can be a tell-tale sign. Exhaust fumes that are white (when above 30-40 degrees), black, or blue all need to be inspected. White smoke is typically a sign of water and antifreeze. Black smoke is a sign of improper air-fuel ratio, while blue smoke could be a sign of burning oil.

Replacing your catalytic converter can ensure efficient operation of your car’s exhaust system. You may also have to replace the oxygen sensor. After you’ve replaced or repaired these two components, check your OBD-II scanner again to ensure you’ve completely solved the OBD-II scanner error code P0420 or P0421 problem.

Advice, how-to guides, and car care information featured on AutoZone.com and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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