P0420 – Catalytic converter system, bank 1 -efficiency below threshold

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By Reinier (Contact Me)
Last Updated 2023-03-13
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0420 Catalytic converter system, bank 1 -efficiency below threshold
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Catalytic converter, wiring, HO2S 2

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Table of Contents

  1. What Does Code P0420 Mean?
  2. Where is the P0420 sensor located?
  3. What are the common causes of code P0420?
  4. How expensive is it to fix code P0420?
  5. What are the symptoms of code P0420?
  6. What are common solutions to code P0420?
  7. How serious is code P0420?
  8. How safe is it to still drive the car with code P0420?
  9. How difficult is it to repair code P0420?
  10. What are the common mistakes when repairing code P0420?
  11. How do you troubleshoot code P0420?
  12. Codes Related to P0420
  13. Frequently Asked Questions
  14. Get Help with P0420

What Does Code P0420 Mean?

OBD II fault code P0420 is a generic code that is defined as “Catalyst System Efficiency Below Threshold (Bank 1)”, and is set when the PCM (Powertrain Control Module) detects a condition that indicates that the catalytic converter on Bank 1 is operating below the minimum allowable efficiency threshold. Note that on engines with two banks of cylinders, “Bank 1” refers to the bank of cylinders that contains cylinder #1.

The purpose of the catalysts in a catalytic converter is to break down harmful exhaust emissions, such as oxides of nitrogen (NOx) and others, by a process of oxidization that depends on complex chemical reactions that are controlled (or triggered) by the catalytic metal(s) in the converter. However, this process can only be initiated when the catalytic metals in the converter are at a very specific minimum temperature, which varies according the converter design, and the type of catalyst used.

While catalytic converter designs have evolved greatly from when they were first introduced, their basic operating principles remain the same. Essentially, a catalytic converter contains a honeycomb, or other type of structure (aka “substrate) that allows the exhaust stream to pass through the body of the converter. Depending on the design of the converter, the substrate is coated with a variety of precious metals that act as catalysts when the exhaust stream passes through the substrate.

NOTE: Catalysts are substances that are not consumed in the process of transforming chemical substances or compounds into another substance or compound, such as turning oxides of nitrogen into water, oxygen, and other innocuous substances.

It must be noted though that while catalytic converters are designed to have long service lives, there are many factors that can severely reduce both its efficiency and service life. Typical examples would be using unsuitable or poor quality fuel, excessive oil consumption, persistent misfires, over fuelling, and several others. Thus, to be able to monitor the efficiency of a catalytic converter, the PCM uses input data from both the upstream and downstream oxygen sensors, or on some applications, the air/fuel ratio sensors.

In a fully functional system, the upstream oxygen sensor both controls and monitors short-term fuel trims by reacting to changes in the levels of oxygen in the exhaust stream, while the downstream sensor monitors the efficiency of the catalytic converter, and is (with a few exceptions) not directly involved in fuel trim control. While the signal voltage from the upstream oxygen sensor fluctuates between about 0.1 volt and .9 volt several times per second, the signal voltage reading from the downstream sensor will remain fairly constant at around the midway mark, provided the throttle position does not change.

In practice, the PCM continuously compares the signal voltages from the upstream and downstream sensors, taking into account the expected variations in the composition of the exhaust stream during normal driving. However, should the PCM detect that the signal voltage pattern from the downstream oxygen sensor closely matches that of the upstream sensor, it will interpret this condition to mean that the catalytic converter is operating below a minimum allowable efficiency threshold. When this happens, the PCM will set code P0420, and illuminate warning light.

Video 1: P0420 – 2004 Honda Civic Si 2.0L (Part 1 Diagnosis)
Video 2: P0420 – 2004 Honda Civic Si 2.0L (Part 2 Replacement)

Where is the P0420 sensor located?

Catalytic converters generally resemble exhaust mufflers, and they are fitted into the exhaust system upstream of the actual mufflers. The image below shows the typical position of a catalytic converter on Bank 1, as well as the locations of the #1 and #2 oxygen sensors. Note the arrow that indicates the direction of gas flow.

What are the common causes of code P0420?

The most common causes of code P0420 are many and varied, but could include the following-

  • Catalytic converter failure due to long use
  • Premature failure of rebuilt or refurbished catalytic converters
  • Failures and/or malfunctions in almost any system, sensor, or component that affects/controls/monitors/regulates combustion, and therefore, exhaust emissions
  • Use of unsuitable or poor quality fuel
  • Exhaust leaks in exhaust components such as exhaust manifolds, down pipes, balancing tubes, flexible hoses/pipes, and mufflers
  • Restrictions anywhere in the exhaust system that prevents or inhibits the free flow of exhaust gas
  • Mechanical issues in the engine, such as excessive wear of cylinders/piston/piston ring/valve gear that cause excessive oil consumption
  • Use of enhanced or unauthorized PCM programming
  • Unauthorized or illegal exhaust system modifications or alterations
  • Failed or failing PCM. Note that in the case of code P0420, this is an exceedingly rare event and the fault MUST be sought elsewhere before any control module is replaced

How expensive is it to fix code P0420?

As a general rule, the cost of a catalytic converter replacement runs to about $2000, parts and labour included. Note though that this estimate does not include taxes and fees that may be payable, nor does it include the cost of resolving underlying issues that may have contributed to the failure.

WARNING: Be aware that while rebuilt catalytic converters can be had for a fraction of the cost of an OEM grade unit, the savings thus made is a false economy, since the “rebuilt” converter can fail at any time, and usually within the first few thousand miles of use.

What are the symptoms of code P0420?

Some common symptoms of code P0420 could include the following-

  • Stored trouble code, and an illuminated warning light
  • Depending on the root cause of code P0420, other codes relating to misfires, fuel pressure, engine temperature, oxygen or air/fuel ratio sensors, and others may be present
  • Vehicle may not pass an emissions test
  • Hard, or no start conditions may be present if the catalytic converter had suffered significant damage
  • Depending on the degree of damage suffered by the converter, the engine may idle roughly or not at all, or the engine my exhibit varying degrees of power loss, particularly upon acceleration
  • Fuel consumption may increase considerably in some cases
  • In some cases, and especially where the converter had suffered significant damage, the converter may overheat to the point where it can set the vehicle on fire
  • In many cases, but depending on the nature of the problem, there may be no discernible symptoms apart from a stored code and an illuminated warning light

What are common solutions to code P0420?

Common solutions to code P0420 could include the following-

  • Resolving all possible underlying conditions that could affect exhaust emissions- refer to the “Common mistakes” and “Causes of code P0420” sections of this guide for more details.
  • Restoring PCM programming to stock
  • Removing unauthorized or illegal exhaust system modifications
  • Replacing substandard rebuilt catalytic converter(s)
  • Removing restrictions from the exhaust system

How serious is code P0420?

Code P0420 should be considered as serious even in the absence of obvious driveability symptoms, since the root cause of the code is not always immediately apparent. If left unresolved, this code has the potential to cause serious, if not fatal damage to the catalytic converter, which in turn, could set the vehicle on fire. Ideally, the vehicle should not be driven until code P0420 had been resolved.

How safe is it to still drive the car with code P0420?

As stated above, applications that display code P0420 should not be driven due to the distinct possibility that the vehicle could be immobilized, or worse, catch fire.

How difficult is it to repair code P0420?

Diagnosing this code should be well within the capabilities of most non-professional mechanics, provided a scanner that can monitor live data streams is available, and that there are no other codes present.

However, if the application uses air/fuel ratio sensors and additional codes relating to these sensors are present, an oscilloscope and a waveform library might be required to diagnose the cause of the sensor failure(s). If such equipment is not available, the better option would be to refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair.

NOTE #1: Bear in mind that while diagnosing this code might not be too difficult, replacing a catalytic converter requires partial or sometimes complete disassembly and/or removal of sections of the exhaust system, as well as some cutting and welding of the exhaust system. This is generally very difficult to do unless a vehicle hoist is available to be able to gain access to the exhaust system.

NOTE #2: Air/fuel ratio sensors and oxygen sensors work on vastly different principles, and while oxygen sensor faults can often be diagnosed with nothing more than a digital multimeter, diagnosing defective air/fuel ratio sensors requires the use of an oscilloscope, or professional-grade diagnostic equipment that incorporates an oscilloscope.

NOTE #3: An infrared or laser based thermometer is a required item when diagnosing this code, since the efficiency of a catalytic converter is closely related to its temperature when the system is operating in closed loop mode.

NOTE #4: One other essential item required to diagnose this code accurately is a gas pressure gauge that is designed to register up to 10 PSI, and is graduated in the smallest possible increments for increased accuracy. This gauge is required to measure exhaust backpressure as a diagnostic aid, but make sure that the gauge is provided with a fitting that will screw into the exhaust in place of the oxygen or air/fuel ratio sensor(s).

What are the common mistakes when repairing code P0420?

Mistakes made when diagnosing this code commonly include the following-

  • Not verifying whether oxygen or air/fuel ratio sensors are in fact defective, or operating correctly before replacing them. These sensors are very often replaced in error when the root cause of the code can involve failures and/or malfunctions in almost any system, sensor, or component that affects/controls/monitors/regulates combustion, and therefore, exhaust emissions.
  • Failing to resolve additional codes in the order in which they were stored. In fact, it is rare for code P0420 to appear by itself, and especially if the application is fitted with an OEM catalytic converter that is less than 100 000 miles old, which distance represents the federally mandated warranty period of OEM catalytic converters.
  • Failing to verify whether the affected catalytic converter on the application is an OEM unit, or a “rebuilt” unit. While refurbished catalytic converters are much cheaper than OEM units are, these rarely if ever, provide the same durability or efficiency as OEM equipment.
  • Failing to verify that the application’s PCM is not running on unauthorized or so-called “performance” settings. “Performance tunes” almost always result in over-fuelling conditions that stock catalytic converters are not designed to deal with over extended periods.
  • Failing to ensure that the engine is in perfect running condition. Before attempting to diagnose/repair this code, it is essential to ensure that there are no misfires, engine vacuum leaks, exhaust system leaks, clogged or damaged mufflers, excessive oil consumption, or other issues present that have negative impacts on the combustion process, or on the engine’s overall operation.

How do you troubleshoot code P0420?

NOTE: Before starting a diagnostic procedure for this code, check to see if the affected catalytic converter is still covered by the federally mandated 100 000-mile warranty. If it is covered, refer the vehicle to the dealer to discuss repair/replacement of the converter under warranty conditions.

Step 1

If the converter is not covered by warranty, record all fault codes present, as well as all available freeze frame data. This information can be of use in tracing the root cause of the code in cases where multiple additional codes are present.

NOTE: Refer to the “Common mistakes” section of this guide for more details on the possible causes of code P0420. If other codes are present, both active and pending, be sure to resolve all additional codes strictly in the order in which they were stored to avoid a misdiagnosis, and the possible unnecessary replacement of a hugely expensive catalytic converter.

Step 2

If no additional codes are present, clear code P0420, and operate the vehicle normally for a few miles to allow the PCM to enter closed loop operation, before scanning the system again to see if the code returns.

NOTE: “Closed loop” operation refers to a condition in which the PCM uses feedback data from the oxygen or air/fuel sensors to control fuel delivery.

Step 3

If the code persists, jack the vehicle up off the ground, and use properly rated jack stands to support the vehicle. This is required to gain (relatively easy) access to the catalytic converter. Once the vehicle is supported safely, start the engine and allow it to idle.

Use the scanner to monitor the operation of the oxygen or air/fuel ratio sensors to verify that all sensors are working as intended. Compare the displayed output signals from the sensors with the values stated in the manual as a double check to ensure that the sensors are not contributing to code P0420.

NOTE: Step 3 assumes that there are no additional codes present, as well as that additional codes that were present had been resolved and cleared, and can therefore no longer influence or affect the operation of the catalytic converter.

Step 4

Once it is certain that the PCM is in closed loop operation, use the thermometer to obtain a reading of the converter’s temperature, but bear in mind that most converters will only start to function at between 400 deg F and 600 deg F (204 deg C and 315 deg C).

If the observed reading falls within this range, have an assistant increase the engine speed to 2 500 RPM, while you are monitoring the converter’s temperature. If the converter is functioning correctly, its temperature will rise as the composition of the exhaust gas changes, but note that the converter is defective if its temperature approaches, or exceeds 2 000 deg F ( 1 100 deg C).

NOTE #1: The normal operating temperature of a functioning catalytic converter is between 1 200 deg F and 1 600 deg F (650 deg C and 870 deg C).

Next, obtain a temperature reading at the inlet of the converter, and then at the outlet. If the engine is in perfect running condition and there are no unresolved additional codes present, the difference between the inlet and outlet temperatures should generally well below 100 deg F (37 deg C). Note though that on many applications, and especially those with multi-point injection systems, this difference may be all but undetectable with DIY equipment.

However, if the temperature difference approaches or exceeds about 200 def F (93 deg C), or goes as high as 500 deg F (260 deg C), it is likely that the converter has suffered damage caused by the long-term effects of one or more underlying conditions, even if there were no additional codes present at the time that code P0420 appeared.

NOTE: A common clue of catalytic converter overheating is a deformed or discoloured converter shell. However, note that code P0420 can appear even if the converted has never overheated, such as in cases where the converter is no longer operating at peak efficiency due to long use.

Step 5

Resist the temptation to condemn the catalytic converter if higher than normal temperature reading are observed, since excessive temperatures are often caused by restrictions in the exhaust system. Check this as follows, but wait for the exhaust system to cool down to avoid burns and scalds-

  • Remove the upstream oxygen or air/fuel ratio sensor, and screw the pressure gauge into the exhaust system in its place. Start the engine and allow it to idle while you are observing the exhaust backpressure. Note the pressure reading, and compare this value with the maximum allowable value stated in the manual. Note that this value varies greatly between applications; therefore, there is no single value that is valid for all applications. If the actual backpressure exceeds the maximum allowable value, there is a restriction in either in the converter, or somewhere else in the exhaust system.
  • To narrow down the location of the restriction, refit the upstream sensor, and screw the pressure gauge into the exhaust system in the place of the downstream sensor, and take a pressure reading with the engine idling. If the measured backpressure remains the same, the restriction is in one or more mufflers: however, if the backpressure now falls into the acceptable range as specified by the manual, the restriction is in the catalytic converter.

NOTE: Step 5 assumes that there are no leaks in the exhaust system. If exhaust leaks are present, repair them to industry standards before repeating the backpressure test to avoid a misdiagnosis.

Step 6

If no excessive catalytic converter temperatures are found and the exhaust backpressure falls within acceptable limits, it is almost certain that the converter has failed due to long use. This can be confirmed by the fact that its temperature remains below 400 deg F (200 deg C) even after expended periods of driving.

As stated elsewhere in this guide, replacing a catalytic converter almost always requires equipment and skills that most non-professional mechanics do not possess or have access to. Thus, if a defective catalytic converter is diagnosed, the better option is to refer the vehicle to a specialist exhaust fitment shop for professional assistance.

NOTE: Take note that while the cods listed below are not strictly related to P0420 – “Catalyst System Efficiency Below Threshold (Bank 1)”, these codes often appear with P0420.

  • P0421 – “Warm Up Catalyst Efficiency Below Threshold (Bank 1)”
  • P0422 – “Main Catalyst Efficiency Below Threshold (Bank 1)”
  • P0423 – “Heated Catalyst Efficiency Below Threshold (Bank 1)”
  • P0424 – “Heated Catalyst Temperature Below Threshold (Bank 1)”

Frequently Asked Questions

Will catalytic converter cleaner fix P0420?

The short answer to these questions is simple. No.

Any chemical you put into the gas tank and allow to purse its way through the fuel system, then through the combustion process, is unlikely to have any effect whatsoever on the exhaust system. Not enough of any cleaner left to do any good. And that is only if the reason the catalyst efficiency is low is due to a carbon coated catalyst.

P0420 comes up because the upstream O2 sensor and the downstream O2 sensor activity is getting closer to each other. In a functioning system, the downstream O2 sensor should be almost a flat line voltage, with very little movement. The upstream O2 should be very active, with swings between rich and lean occurring multiple times a second. If this is happening, then the catalyst is working properly.

Read More: Catalytic Converter Cleaning, and Other Scams…

Does P0420 affect performance? 

The code by itself doesn’t affect either performance or mileage. The reasons why the code set can though.

If the reason the code set is due to an overheated plugged catalyst, then yes it can affect both performance and mileage, due to the exhaust restriction.

If the catalyst is carbon coated due to an overly rich condition, then yes, both performance and mileage will suffer.

Could P0420 be an O2 sensor?

In a functioning system, the downstream O2 sensor should be almost a flat line voltage, with very little movement. The upstream O2 should be very active, with swings between rich and lean occurring multiple times a second. If this is happening, then the catalyst is working properly.

O2 sensors are tested by how they respond to rich and lean conditions, and how quickly they do so.

Usually, if an O2 sensor is bad or is going bad, a corresponding fault code for the O2 sensor failure is produced.

Will P0420 cause limp mode?

Typically, it does not cause the vehicle system to go into limp in.

What will cause the vehicle to go into a limp in mode is if another part of the system has caused catalyst damage, such as an overly rich mixture, or an exhaust leak at the catalyst allowing additional air into the system possible overheating the catalyst, or causing the system to compensate for the exhaust leak by adding additional fuel.

How long do catalytic converters last?

Catalytic converters typically last the life of the vehicle, in excess of 150,000 miles.

Many original catalysts have been on the vehicle when it was scrapped in a wrecking yard.

So why are so many replaced? And why are so many being replaced a second time?

If the vehicle is not being maintained, then the items on the vehicle that can cause catalyst damage are not addressed before failure. Things such as any coolant leak into the exhaust system (head gasket leak, intake leaks into the engine, etc) will damage the catalyst. So if the engine has constant problems like these, then catalyst damage will be the end result.

One last thing; to prevent repeat catalytic converter failures, insure you use OEM quality NEW catalytic converters. Getting one from a wrecking yard is like buying the last owners’ problems. Just don’t do it, no matter how much cheaper it is. Unless you like to replace catalytic converters.

How common is P0420?

P0420 is the most common OBD2 code.  As any vehicle ages, the likelihood of a P0420 code becomes more and more likely. If the vehicle is maintained properly and any issue that arises in the fuel control system is addressed in a timely fashion, the likelihood of having a P0420 in your future is slim.

No specific make is more prone to P0420 than any other. As long as there are no recurring fuel control issues or coolant leaks/incorrect additives/incorrect engine gasket adhesives (O2 sensor safe) utilized, then there should be no reason for concern.

We have manufacturer specific articles on P0420 for the following makes:

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