P0420 Code: An In-depth Analysis of Meaning, Causes, Symptoms, and Fixes

The automotive manufacturers keep adding new features to newer cars, and this has also changed their impact on the environment. Older cars release harmful gases into the environment from the exhaust tailpipes.

Manufacturers saw this as an urgent issue and introduced the catalytic converter to fix these issues. The primary job of the catalytic converter is to convert harmful gases from the combustion chamber into less toxic ones before releasing them into the atmosphere.

The work of the catalytic converter is so important because if they fail, the vehicle will release harmful gases into the atmosphere and cause the car to fail emission tests. Each bank of the engine has two oxygen sensors on the exhaust pipes.

These sensors monitor and measure the level of pollutants passing through the exhaust and signal the powertrain control module. For example, if the O2 sensors in bank 1 detect that the catalytic system efficiency in bank 1 is below a threshold, they’ll signal the ECM. And the ECM will register a P0420 on the memory.

p0420 code

What does error code P0420 mean?

A P0420 shows the PCM has detected an issue with the catalytic converters. As discussed above, the catalytic converter converts toxic gas from the combustion chamber into less harmful gases before releasing them into the atmosphere. They achieve this by using rhodium, gold, platinum, and palladium mesh filters.

If the Upper O2 sensor is functioning properly, it should send a fluctuating signal from the vehicle is running in a closed loop and when at operating temperature. However, the reading will be steady when the lower oxygen sensor is working properly, and the catalyst converter is okay.

If both the upstream and downstream have the same or similar readings, it shows the catalytic converter is malfunctioning. If the downstream sensor starts fluctuating like the upstream, it indicates there’s too much oxygen in the engine. And the engine control unit (ECU) will register P0420 and display a check engine light to notify the driver of imminent issues in the engine.

What is the symptom of error code P0420?

 A P0420 on ford or any car model poses no noticeable signs of a bad catalytic converter and little or no drivability issues. However, there are a few symptoms that indicate the PCM has thrown the P0420 error code.

Check engine light

Several parameters can cause the car computer to project the check engine light on the dashboard. And a P0420 code bank 1 is no exception. When the ECM detects the catalytic converters system is below the threshold in bank 1, it’ll display the check engine light to notify the driver of imminent issues in the system.

However, you’ll need to diagnose your vehicle when the check engine light pops on the dashboard because you can’t just say what the problem is by just looking at the warning light.

The car may not exceed a certain speed level

Depending on the severity of the P0420 on Daihatsu or your respective car model, the car may not exceed 30-40 mph. So unless you’re driving on a freeway or highway at a higher speed, you may not notice it. 

Lack of power

When the powertrain control module detects issues in the bank 1 catalytic converter, the engine will not perform efficiently as the engine warms up. For instance, you will notice a loss of engine power when driving.

Rotten egg smell

A rotten egg smell is a common sign of a bad catalyst converter. This foul smell is created due to the improper amount of oxygen in the exhaust system, leading to excess sulfur in the gas tank and creating the awful rotten egg smell.

Other symptoms you may notice include.

What Causes error code P0420?

 Many factors can cause the car computer to register DTC P0420 on Toyota or any car model. This makes it challenging to diagnose and fix the problem.

How serious error code P0420 is?

The P0420 error code is a moderate case. While this is not a severe case, it can cause several highway issues, especially when overtaking a vehicle. The sad story is that most drivers won’t know they have P0420 issues until they try to overtake a car on the highway.

If the catalytic converter issues occur, they may seem common. For instance, if you do city driving, you will hardly notice the vehicle has acceleration issues. But catalytic converters are expensive, and if you don’t fix the issue on time, it’ll cause damage to related components.

How do you fix P0420? 

Since there are several causes of a P0420 error code, the first approach to fixing the problem is to diagnose and track the root cause. Here are some things you can do to repair the issues.

  • Replace catalytic converter (commonly the course)
  • Diagnose and fix any engine misfire issues
  • Repair leaks in the muffler or replaces the muffler
  • Fix leaks in the exhaust pipes or replace the exhaust pipes
  • Fix leaks in exhaust manifolds or replace the exhaust manifolds
  • Replace faulty oxygen sensors
  • Replace bad engine coolant temp sensor
  • Fix or replace bad or leaking injectors
  • Replace or repair oxygen sensor connectors
  • Fix or replace O2 sensor wiring. 

p0420 catalyst system

Common P0420 diagnosis mistakes

The most common mistake amongst mechanics when fixing P0420 error codes is suspecting and replacing parts without a proper diagnosis.

If the powertrain control module registers other codes like P0300, P0171, or P0174, fix the codes alongside the P0420. If not, these issues will also damage the catalytic converter or the oxygen sensors and cause the PCM to trigger P0420 again.

How to diagnose

If you want to fix the P0420 catalyst system efficiency below threshold bank 1 at home and save the service charge, this is the right guide for a simplified diagnosing approach. However, this is not an intermediate or beginner guide. And this is a labor-intensive and time-consuming task.

Parts and Items Needed

  • Scan tool
  • Digital multimeter

Step 1: Diagnose the vehicle

Locate the OBD port on your vehicle and plug in the scan tool. Next, diagnose the vehicle and ensure there are no additional codes on the vehicle. If you find any code aside from P0420, track the root cause and fix it before proceeding to the next step.

Step 2: Inspect the exhaust system

Inspect exhaust components like manifold gaskets, O2 sensors, and exhaust pipes. If you find any leak or damages, repair or replace the faulty parts and erase the fault code. After that, the test runs the vehicle for around 20 to 30 minutes. Next, scan the vehicle and see if the code appears. If the code is still there, proceed to the next step.

Step 3: Test the downstream oxygen sensor

Start the engine and allow it to idle to the average operating temperature. While the engine is still running, test the downstream oxygen sensor with a multimeter and note the voltage reading.

A good downstream O2 sensor should read around 0.45v when the catalytic converter is functioning properly. If the voltage fluctuates between 0.1v to 0.9v, you have a bad catalyst converter that needs cleaning or replacement.

How much does it cost to fix a P0420?

Several factors can cause the P0420 error codes, making it impossible to estimate the repair cost without a proper diagnosis. 

You can drive to an auto repair shop and have them diagnose the root cause of the problem. Most service technicians will charge you for an hour of diagnosis. The average hourly rate for auto repairs is around $75 to $150. After the diagnosis, the service technician can estimate the repair cost and fix the P0420 error codes.

The diagnosis will likely point to one or more of the following parts. The prices are average estimations, meaning they can be higher or lower than the prices here.

  • Air fuel sensor $150 to $200
  • Catalytic converter $500 to $1000
  • Replace oxygen sensor $200 to $300
  • Exhaust leak $150 to $250.


Q: What can trigger a P0420 code?

Several factors can cause the ECM to throw a P0420 error code. Here are the most common reasons.

  • Engine misfire
  • Engine running lean or rich
  • Contaminated oil in the catalytic converter
  • Malfunctioning engine coolant temp sensor
  • Wrong or loose O2 sensor wiring connection
  • Faulty oxygen sensor
  • Bad exhaust manifold
  • Defective exhaust muffler
  • faulty exhaust pipes
  • Leaking fuel injectors
  • Increased fuel pressure

Q: Is it okay to drive with P0420?

It’s okay to drive with a P0420 until you get to your destination or a mechanic shop. Sometimes, you can drive with it for a few weeks until you get money to fix the problem.

However, if you ignore the P0420 error code and continue driving the vehicle for an extended period, it’ll expose the catalytic converter to a catastrophic failure. And the issue can extend to other related components.

Q: Will catalytic converter cleaner fix P0420?

Yes. You can fix the OBD P0420 code by using a catalytic converter cleaner. To do this, get a catalyst converter cleaner and pour the full can into the gas tank. After that, clear the P0420 code bank 1.

Next, drive the vehicle for 30 minutes while maintaining 45mph or above. This should fix the P0420 if a lousy catalyst converter is a culprit.

Q: Will P0420 clear itself?

The error code P0420 on Nissan or any car model will not be clear on its own. You’ll either track the culprit and fix it or clear the error code with a scanning machine for the code to disappear.

However, erasing the code with a scan tool without fixing the problem will not rectify the problem. It’ll only delete the code from the car memory at that particular time. After some time, the code will reappear again.

Q: Can a bad oxygen sensor cause a P0420 code?

Yes. The oxygen sensors monitor and regulate the oxygen in the exhaust gases and relate this information to the car computer. The downstream O2 sensor monitors the efficiency of the catalyst converter, and the upstream signals a reference point.

Therefore, if any of these sensors fail, they can send false readings to the ECU and cause it to throw error code P0420 on SUZUKI or your respective car model.

Final Words

The catalyst converter is an essential component that converts harmful gases from the combustion chamber into less toxic ones before releasing them into the atmosphere. For this reason, we should always ensure they are always working properly.

Unfortunately, nothing lasts forever. They can fail over time and require replacement or fixing. In addition, several other factors can cause the powertrain control module to throw DTC P0420 on Honda or any car model.

Whatever the culprit, track it and fix the P0420 error codes on time to prevent it from causing other system problems.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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