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

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By Reinier (Contact Me)
Last Updated 2018-07-04
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 P420 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 that the catalytic converter on bank 1 is operating below a minimum allowable efficiency threshold. Note that while “Bank 1” refers to the bank of cylinders that contains cylinder #1 on V-type engines, code P0420 also sets on inline 4 & 6 cylinder engines that do not have multiple cylinder heads.

The purpose of a catalytic converter is to convert the harmful components of exhaust gas into harmless substances such as water vapor when the exhaust stream passes over a substrate that is coated with various precious metals, which act as catalysts in the conversion process. Note that to work as expected, the internal temperature of the catalytic converter must to at least 8000F and no misfires, exhaust leaks, and engine vacuum leaks must be present, and oil and fuel consumption must be within specifications. When all required conditions are met, most catalytic converters will typically achieve efficiencies of more than 95%.

To monitor this efficiency, the PCM uses input data from both the upstream and downstream oxygen sensors, and provided both sensors are fully functional and in closed loop operation, the difference between the output data patterns of the sensors represent the efficiency of the catalytic converter. As a practical matter, the signal voltage of the upstream oxygen sensor fluctuates rapidly both as a result of changes in the exhaust stream caused by normal driving, and the fact that the PCM causes the sensor to read rich and lean conditions alternately several times per second. By switching the sensor in this way, the PCM is better able to average out lean and rich conditions in order to establish a 14.7: 1 air/fuel mixture, which is the ideal mixture for gasoline engines.

If the catalytic converter is functioning properly, the output data of the downstream oxygen sensor will hover around the mid-point between a rich and a lean mixture, with only very small variations in this readings to either side of the mid-point. If however, the output data from the downstream sensor tracks the data pattern of the upstream sensor, the PCM recognizes that the catalytic converter is not functioning optimally, since the downstream sensor is registering changes in the air/fuel mixture, which it won’t do if the converter were functioning properly.

In practice, the PCM converts the degree of similarity (or difference) between the data patterns of both oxygen sensors into a percentage that reflects the efficiency of the catalytic converter, and in most cases, a percentage of less than about 75% represents a minimum allowable efficiency threshold. If this threshold is reached, the PCM will set code P0420, and illuminate warning light.

Why does code P0420 affect HONDA applications more than others?

While code P0420 is a very common code across all manufacturers, some Honda models and most notably sixth-generation Honda Accord models from the early 2000’s were plagued by substandard catalytic converters that were known to fail prematurely. In many cases, catalytic converter failure on these models could be postponed, if not prevented by reflashing the PCM with updated software, but how well (or otherwise) this worked depended on the amount of damage the catalytic converter had already suffered by the time the reflash was performed. In addition, many Honda Civic models from the 1995 through 2005 production years were fitted with exhaust manifold/catalytic converter assemblies that cracked/fractured at an average mileage of only 150 000 miles, which caused exhaust leaks that affected the operation of the catalytic converters on these models  in the worst possible way.

The most common causes of code P0420 on late model Honda applications are engine vacuum leaks and fuel trim issues. Since fuel trim percentages/values vary greatly between Honda applications, proper diagnostic procedures must be followed in all instances of P0420 on any Honda application to diagnose and correct fuel trim issues before a catalytic converter is condemned out of hand.

Where is the P0420 sensor located?

The image above shows the location (center of frame) of the catalytic converter on a 2001 Honda Accord model. Note though that on many late model Honda applications, the catalytic converter and the exhaust manifold form an assembly close to the engine, and this assembly must be replaced as a unit when the converter fails.

What are the common causes of code P0420?

Some common causes of code P0420 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring/connectors in the control/signal/heater circuits of any oxygen sensor or air/fuel ratio sensor, but note that these conditions will almost always be indicated by one or more additional codes
  • One or more defective oxygen sensors
  • Defective MAF or MAP sensor
  • Worn sparkplugs, or damaged ignition coil(s)
  • Unresolved misfires or exhaust leaks
  • Restrictions in the exhaust system downstream of the catalytic converter
  • Unmetered air entering the engine
  • Unsuitable, or incorrect fuel
  • Incorrect or unsuitable engine oil
  • Excessive oil consumption due to mechanical issues or excessive engine wear
  • Oil level too high
  • Contamination of the exhaust stream by engine coolant
  • Excessively retarded ignition timing
  • Excessive fuel pressure
  • Defective or leaking fuel injector(s)
  • Defective, clogged, or damaged catalytic converter

NOTE: Since almost all of the possible causes of code P0420 listed here will be indicated by dedicated codes other than P0420, all additional codes must be investigated and resolved in the order in which they were stored. Failure to do this will almost certainly result in a misdiagnosis, and the unnecessary replacement of sometimes-expensive parts and components.

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