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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)