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

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By Reinier (Contact Me)
Last Updated 2023-11-21
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. Get Help with P0420

What Does Code P0420 Mean?

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

The purpose of a catalytic converter is to convert harmful exhaust emissions (mainly oxides of nitrogen) into innocuous substances such as water and carbon dioxide. While design specifics vary between manufacturers, all catalytic converters consist of a substrate that is coated with catalysts in the form of inert metals such as platinum, iridium, palladium, and others. Provided that the catalytic converter is at its optimal operating temperature, which also varies between manufacturers depending on the substrate and metals used, the catalysts convert harmful substances into less harmful ones as the exhaust stream passes over the catalysts.

NOTE: Note that the metals used in catalytic converters are called “catalysts” because they cause and sustain the conversion process, but are not themselves consumed in the conversion process.

However, even though most OEM catalytic converters are designed to have very long service lives, there are many factors at work (see Causes section) in all applications that can seriously affect the efficiency of any catalytic converter. Therefore, the PCM uses a very simple, but relatively effective strategy to monitor the efficiency of all catalytic converters on the application on a continuous basis.

In practice, the PCM uses input data from two primary sensors- the #1 (upstream of the catalytic converter) oxygen sensor, and the #2 (downstream of the catalytic converter) oxygen sensor to calculate an efficiency value for the catalytic converter. As a practical matter, the upstream sensor monitors the composition of the exhaust gas, and the PCM makes adjustments to the fuel trims and ignition timing in direct response to input data received from this sensor, while the downstream oxygen sensor monitors the efficiency of the catalytic converter.

If the catalytic converter is in good condition and functioning properly, the signal from the downstream sensor will be almost constant, as opposed to the rapidly changing signal voltage from the upstream sensor. Thus, when the PCM compares the input data from the two oxygen sensors, it expects to see a difference between the two signal voltages, with the signal voltage from the upstream sensor fluctuating sharply, while the signal voltage from the downstream sensor should be fairly constant.

If however, the PCM detects a similar response from both sensors, it uses the degree of similarity to calculate an efficiency value for the catalytic converter based on how similar the two signal voltages are. In most cases, the PCM will set code P0420 and illuminate warning light when it calculates an efficiency value of between 75%, and 70%, with 70% representing the lowest allowable efficiency threshold on most applications.

So why does code P0420 affect Toyota applications more than others?

Although this code can potentially affect all applications, the fact is that due to their design and construction, catalytic converters on almost all Toyota are far more sensitive to faults, defects, and failures that affect the operation of catalytic converters, than the catalytic converters on other applications are.

In many cases, the PCM’s programming on Toyota applications is to blame in the sense that its calibration is so sensitive that it is almost fault-intolerant, which means that the PCM will set code P0420 long before a similar set of circumstances will trigger this code on most non-Toyota applications. Note that in many, if not most instances of code P0420 on Toyota applications, the problem can be resolved by reprogramming the PCM with updated software.

It should also be noted that Toyota is the only manufacturer that recommends a process of cleaning the affected catalytic converter in a procedure that is roughly analogous to how a DPF (Diesel Particulate Filter) is regenerated. However, this process should only be performed by the dealer or other competent repair facility, but be aware that it has a success rate of less than 50%.

Where is the P0420 sensor located?

The image above shows the typical location of catalytic converters on Toyota applications, as well as the locations of both the upstream and downstream oxygen sensors. Note that although the terms “oxygen sensor”, “and air/fuel ratio sensor” are sometimes used interchangeably, Toyota applications use air/fuel ratio sensors, which are NOT interchangeable with oxygen sensors.

What are the common causes of code P0420?

Bear in mind that if reprogramming the PCM does not resolve code P0420, there are many other possible causes to consider, some of which could include the following-

  • Exhaust leaks
  • Excessive fuel pressure, but note that this will almost certainly be indicated by a dedicated trouble code
  • Defective engine coolant sensor
  • One or more defective(leaking) fuel injectors
  • Loss of sensitivity of the affected air/fuel ratio sensor due to long use
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • Defective or contaminated air/fuel ratio sensor. Typical contaminants include engine oil, carbon, engine coolant, and silicon-based compounds found in some aftermarket gasket sealers, and some aftermarket fuel and oil additives
  • Defective catalytic converter
  • Use of incorrect ,unsuitable, or contaminated fuel
  • Failed or failing PCM or other control module, but note that this is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced.

NOTE: While premature failure of catalytic converter s on Toyota applications is a common occurrence, there are many other possible causes of code P0420 on these applications. Therefore, it is important to record all codes, as well as all available freeze frame data present, and to resolve all additional codes present in the order in which they were stored before condemning the catalytic converter out of hand.

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