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What Is P0420 Code? [Major Causes & Fixes]

There are several problems that might cause the Check Engine Light to come on. However, your catalytic converter may be having problems if you utilize an OBD-II scanner and obtain a P0420 code. Read on for a detailed explanation of what this code involves.

What Does The Engine P0420 Code Indicate?

The catalytic converter in bank 1’s sensor has identified an efficiency below the threshold, according to OBD2 Diagnostic Trouble Code P0420. After combustion, a catalytic converter’s purpose is to burn away any leftover raw fuel and remove or drastically decrease hazardous pollutants. Two oxygen (O2) sensors are used in a catalyst efficiency monitor (CEM), one upstream and the other downstream of the catalyst converter.

The front sensor’s values often change when these sensors are functioning properly, however, the back sensor’s measurements stay consistent. The check engine light (CEL), also known as a malfunction indicator lamp (MIL), will turn on if both sensors show comparable values, which shows that the catalytic converter is not functioning properly. The Powertrain Control Module (PCM) will also store the P0420 issue code as a consequence.

How Dangerous Is Engine Failure Code P0420?

Engine issue P0420 code seldom poses a risk to driving safety. It’s unlikely that the flaw will have a significant impact on dependability or drivability. However, because the catalyst system on bank 1 is malfunctioning, your engine can be spewing out more harmful pollutants. This makes it unlikely that you will pass emissions checks, especially when accompanied by the check engine light.

The safest course of action, like with any engine issue, is to identify and fix the P0420 issue as soon as practical. Therefore, we don’t advise you to put up with these problems for several weeks or months. Instead, you should probably continue driving while the engine has the P0420 error number. Ensure you act morally. If driving the car feels unsafe, there are additional engine codes, significant symptoms, etc., drive safely.

Is it Safe to Drive With P0420?

In most cases, it is safe to keep driving while having P0420. The motorist is not at risk because of it. Long-term use of this code while driving might harm your engine and exhaust system. In spite of the fact that you don’t need to contact a tow truck if you notice this code, you should try to identify and resolve the issue as soon as you can.

Your car’s performance might noticeably deteriorate. This is a result of your exhaust system’s subpar performance. If other codes that indicate an excessive fuel level in your air/fuel combination are present, P0420 might become more dangerous. See whether there are any other codes, such as P0171, P0172, P0174, or P0175. These errors might indicate that the engine is receiving the incorrect fuel-to-air ratio.

Symptoms of P0420 Code

Even though a failing catalyst system often doesn’t cause any obvious symptoms, including difficulties with drivability, there are four typical markers for the P0420 code:

  • A rotten Egg Smell Comes From The Exhaust Pipe

The exhaust system smells when catalytic converters don’t correctly convert oxygen, which leads to an excess of sulfur building up in the fuel tank.

  • The maximum speed of the vehicle is 30 to 40 mph

You might not feel the symptom if you drive more slowly in primarily residential neighborhoods, but you might on motorways and motorways with greater speed restrictions.

  • The Vehicle Lacks Power After Warming Up

It will be difficult to accelerate as rapidly, and the engine performance will be subpar.

  • The check engine light is on

The check engine light may come on for a variety of reasons. A mechanic needs a diagnostic instrument to identify the root of the issue.

How To Diagnose The P0420 Error Code?

Here is a brief explanation of how to identify and resolve the P0420 engine code:

  • First, confirm that P0420 is the sole engine code present.
  • Remove the code, then drive for around 15 minutes. Examine the code to check whether it is returned. If not, do it again a few times to be certain. When the DTC P0420 displays, a real issue exists.
  • Physically inspect the exhaust system for any holes, corrosion, damage, or other possible leaks. Remember to check the exhaust manifold, which is a popular place for leaks.
  • Check the O2 sensor’s voltage value. Standard readings for the majority of engines should be between 0.4 and 0.5 volts. The catalytic converter is most likely to blame if it is noticeably off or jumping about.
  • Occasionally, engines have small issues, and fault codes appear at random. In these situations, just clearing and removing the code could be enough to fix the problem. If not, it’s time to identify the cause of DTC P0420. The methods described above are often effective for fixing exhaust leaks and catalyst failure. But you’ll need to use a bit more imagination and problem-solving skills if you’re getting more engine error codes.

What Are the P0420 Code’s Potential Causes?

The most common reason for a P0420 error code is a damaged catalytic converter. The following problems can result in this OBD-II code:

  • A malfunctioning catalytic converter
  • An unreliable oxygen sensor.
  • Issues with the circuit, such as frayed wires and loose connections.
  • A leaky exhaust.
  • Problems with computers.

Various additional issues might potentially cause the catalytic converter to malfunction. These include problems with the way the engine performs, including a misfire or an incorrect air-to-fuel ratio, which can lead to an overheated converter. The P0420 error code may be caused by damaged wiring or loose connections. The cat can also be harmed by pollutants like coolant and motor oil.

Ensure That It Is the Only Code

You must deal with any other codes first since they could be the origin of the p0420 error. To check for additional codes, you will need an OBD2 scanner like FIXD. Other codes like P0300 or P0171 might be the cause of the issue if they are present. Without first clearing these codes, you should not replace the catalytic converter. Your new catalytic converter will malfunction again if you install it without fixing the other code.

How Do I Repair Exhaust Leaks With P0420?

The common error number P0420 is brought on by exhaust leaks. First, look for any obvious damage to the exhaust system. A straightforward and affordable answer may be welding. However, it will probably just be a temporary fix if the overall quality of your exhaust is poor. The need for a new exhaust system, exhaust manifold, etc. may then become apparent.

How Much Does It Cost to Repair Code P0420?

Depending on the problem, repairing P0420 might cost anywhere from $0 to $2400. The price ranges for having a mechanic handle typical repairs are listed below:

  • $200-$300 for an air-fuel sensor replacement;
  • $275-$500 for an oxygen sensor replacement;
  • $400-$2400 for a catalytic converter repair.
  • A $100-$200 exhaust leak if it’s just a weld.

You can do it yourself for a lot less money, but if you’re inexperienced, you run the danger of doing more harm.

Make sure you accurately identify the source of the code before attempting to repair P0420. You won’t have to pay for it if you remove the code and it doesn’t return after you’ve driven. You won’t have to spend a lot of money on the repair if the exhaust system has a little leak that can be repaired with welding. However, replacing the catalytic converter would be a pricey repair if you need to.


To prevent harm to the catalytic converter, the ignition system, fuel system, air intake, and misfire issues must be fixed as quickly as possible. P0420 error codes are frequently caused by these parts.

When changing a catalytic converter, it is preferable to use an OEM component or a premium oxygen sensor replacement.

In the event that an aftermarket oxygen sensor fails, which happens frequently, the P0420 error code can reappear. To find out if your vehicle’s emissions-related components are covered by a manufacturer’s warranty, it’s a good idea to get in touch with the manufacturer.

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