Catalytic Converter, Oxygen Sensor

Catalytic Converter and Oxygen Sensor Issues

Separating fact from fiction can be a daunting task for any parts professional. To illustrate, many DIYers and professional mechanics might believe that changing an oxygen sensor will turn off the “check engine” light on a vehicle with a defective catalytic converter.

Separating fact from fiction can be a daunting task for any parts professional. To illustrate, many DIYers and professional mechanics might believe that changing an oxygen sensor will turn off the “check engine” light on a vehicle with a defective catalytic converter.

Others believe that the catalytic converter must always be replaced when a P0420 or P0430 diagnostic trouble code is stored. Others believe that expensive catalytic converters can be replaced with cheaper substitutes. These are issues that any parts professional might face on a daily basis, especially in localities that enforce strict emissions inspection standards. To better understand how catalytic converters are diagnosed and replaced, let’s begin by discussing how an OBD II catalytic converter changes toxic exhaust fumes into gases found naturally in the atmosphere.


Fossil fuels, like gasoline, belong to a chemical family called hydrocarbons, which is expressed by the chemical symbol “HC.” Hydrocarbons are various combinations of hydrogen (H) and carbon (C) that, when burned inside an engine’s cylinders, combine with air, which consists of about 78 percent nitrogen, 21 percent oxygen and 1 percent other gases.
If the air/fuel mixture entering the engine is a chemically perfect ratio (ex. 14.7 grams of air to 1 gram of fuel) and complete combustion took place, the exhaust gases exiting the engine will consist primarily of water (H2O), carbon dioxide (CO2), nitrogen (N) and some oxygen (O).

But the process of internal combustion is an imperfect method of oxidizing hydrocarbons. Not all of the HC in gasoline is combined with all of the atmospheric O2 inside an engine’s cylinders because a small volume of unburned HC and O2 remains in a very thin boundary layer of air/fuel mixture located at the surface of the combustion chamber. This small amount of unburned hydrocarbon or gasoline enters the exhaust gas stream as an exhaust pollutant. The combustion process may also lack sufficient oxygen and the product is a poisonous exhaust gas called carbon monoxide (CO). Although modern fuel controls have greatly reduced CO emissions, CO remains a highly toxic pollutant in areas with high vehicle populations.

Last, because atmospheric oxygen is composed of approximately 78 percent nitrogen (N), various compounds of nitrogen oxides (Nox) are produced during the combustion process. Although N is normally an inert gas that doesn’t readily combine with oxygen, it will form a false chemical bond with oxygen under a combination of high pressures and temperatures, all of which occur during the process of internal combustion. The presence of Nox in the atmosphere forms a photochemical smog when exposed to sunlight and humidity.

HC, O2, CO, and Nox form the majority of exhaust gases flowing into the catalytic converter. By definition, catalysts accelerate chemical reactions without themselves being changed or otherwise affected by that same chemical reaction. A catalytic converter breaks down gaseous exhaust compounds into their basic chemical components by exposing the gases to precious metal catalysts like platinum, palladium, rhodium and cerium.

For the 1996 model year, the federal Environmental Protection Agency (EPA) mandated that auto manufacturers design catalytic converters that will reduce all exhaust emissions into their component parts of water, carbon dioxide, nitrogen and oxygen, which are elements found naturally in the earth’s atmosphere.

In practically all cases, the “check engine” warning light comes on when a mechanical or electronic malfunction causes exhaust emissions to exceed by 1.5 times the Federal Test Procedure (FTP) standard. Early pre-1996 OBD I catalytic converters were only designed to break down hydrocarbons and carbon monoxide. After 1996, OBD II converters were also designed to break down nitrogen oxide (Nox) into its component parts of nitrogen (N) and oxygen (O).

To meet the On-Board-Diagnostics II (OBD II) standards introduced in 1996, auto manufacturers devised a method to evaluate catalytic converter performance. To accomplish this, auto manufacturers installed an oxygen sensor at the inlet (upstream) and at the exit (downstream) of the catalytic converter. The PCM then switches the air and fuel mixture entering the engine from rich to lean or from high-voltage to low-voltage. The upstream oxygen sensor relays this voltage signal back to the PCM indicating that the air/fuel mixture is switching rich/lean.

In the voltage graph above, the engine is warming up and the upstream oxygen sensor (top) begins sending a switching voltage signal to the PCM. As the catalytic converter warms up, the downstream oxygen sensor (bottom) changes from a switching to a constant-voltage signal.

The signal from the downstream oxygen sensor indicates when the catalytic converter is operating efficiently. If the catalytic converter is breaking HC, CO, and Nox into their component parts, the downstream oxygen sensor will display a constant voltage.

In the graph at right, the downstream oxygen sensor voltage stabilizes at about .750 volts, which indicates that the catalytic converter is working efficiently.

When a catalytic converter becomes defective, the signal from the rear oxygen sensor begins to duplicate the switching signal of the front oxygen sensor. The PCM then uses a mathematical algorithm to determine when the downstream signal exceeds FTP standards. If the downstream signal exceeds FTP standards, then the PCM will illuminate the “check engine” light and store one or more diagnostic trouble codes. The algorithm programmed into the PCM is a far more accurate method of measuring catalytic converter performance than any externally applied method.

If the vehicle has an in-line engine equipped with one catalytic converter, the PCM will store a P0420 diagnostic trouble code (DTC) indicating that converter efficiency is below FTP standards. If the vehicle is equipped with a V-type engine, a P0430 DTC may also be stored if the bank No. 2 converter fails. These DTCs will turn on the check engine light and can be retrieved by using a code reader or professional scan tool.

Because of the vastly improved on-board diagnostic ability of modern PCMs, the PCM can diagnose a defective oxygen sensor more accurately than can a mechanic armed with hand-held test equipment. To illustrate, the PCM will store a diagnostic trouble code and turn on the “check engine” light if the switching action of the oxygen sensor is too slow or the average switching voltage is too high or low. Can replacing one or both oxygen sensors prevent a P0420 or P0430 catalytic converter trouble code? I won’t say it’s not possible, but it’s highly unlikely.

In cases where a catalytic converter or oxygen sensor fails for no apparent reason, the original diagnostic criteria built into the PCM must be updated or recalibrated to meet real-world operating conditions. The need for recalibration can by done only by checking technical service bulletins for the vehicle application.

The EPA mandates that the two most expensive components of the emissions control system — the PCM and the catalytic converter — be warranted and replaced free of charge by the auto manufacturer during the first eight years or 80,000 miles of service. If the OE converter fails under warranty, the PCM might need recalibrating as described previously to prevent a repeat failure.

In some rare cases, the substrate of a converter can become coated with soot, which might result in a P0420/430 DTC. Because correcting the cause of the soot condition might allow the converter to restore itself to normal efficiency, replacement might not be necessary.

Aftermarket replacement converters generally carry a much shorter warranty than the original “8/80” converter. Keep in mind when selling an aftermarket converter that all trouble code conditions contributing to the failure, such as an engine misfire, must be corrected when the converter is installed. In addition, all technical service bulletins (TSBs) issued by the auto manufacturer pertaining to catalyst failures must be reviewed for required reprogramming or recalibration updates. Because a vehicle’s PCM is programmed to monitor converters designed for specific applications, never try to substitute another part number. In any case, selling replacement catalytic converters can be much less of a headache when everybody concerned relies on fact rather than fiction.

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry.  Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He is also a graduate of Colorado State University and belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).

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