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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.

Gary Goms

7/20/2010

Separating fact from fiction can be a daunting task for any parts professional. To illustrate, many DIYers

and profess ional mechanics might believe that changing an oxygen sensor will turn off the checkengine 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

subs titutes. 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 diagnos ed and replaced, lets begin by discussing how an OBD II catalytic converter

changes toxic exhaust fumes into gases found naturally in the atmosphere.

FUMES IN, DAISIES OUT

Fossi l 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 ins ide an engines cylinders, combine with air, which consists of about 78 percentnitrogen, 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 wi ll 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 engines 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 combus tion chamber. This s mal l 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

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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 doesnt readily combine with oxygen, it will form a false chemical bond with

oxygen under a combination of high press ures and temperatures, all of which occur during the process

of internal combus tion. The presence of Nox in the atmosphere forms a photochemical smog when

exposed to sunlight and hum idity.

CONVERTING EXHAUST GASES

HC, O2, CO, and Nox form the majority of exhaus t gases flowing into the catalytic converter. By definition,catalysts accelerate chemical reactions without themselves being changed or otherwise affected by that

sam e chemical reaction. A catalytic converter breaks down gas eous 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 des ign catalytic converters that will reduce all exhaust emiss ions into their component

parts of water, carbon dioxide, nitrogen and oxygen, which are elements found naturally in the earths

atmosphere.

ON-BOARD DIAGNOSTICS

In practically all cases, the check engine warning light comes on when a mechanical or electronic

malfunction causes exhaust em iss ions 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 m ixture 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 m ixture is s witching rich/lean.

In the voltage graph above, the engine is warming up and the upstream oxygen sensor (top) beginssending a switching voltage signal to the PCM. As the catalytic converter warms up, the downstream

oxygen sensor (bottom) changes from a s witching to a constant-voltage signal.

The signal from the downs tream 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 s ignal from the rear oxygen s ensor begins to duplicate

the switching s ignal 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 FTPstandards, 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 s tore 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 wi ll turn on the check engine light and can be retrieved by using a code reader or profess ional

scan tool.

OXYGEN SENSOR DIAGNOSIS

Because of the vastly improved on-board diagnostic ability of modern PCMs, the PCM can diagnose a

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defective oxygen sensor more accurately than can a m echanic armed with hand-held test equipm ent. To

illus trate, the PCM will s tore 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 wont

say its not possible, but its highly unlikely.

In cases where a catalytic converter or oxygen sensor fails for no apparent reason, the original

diagnostic criteria built into the PCM mus t 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.

WHEN A CONVERTER FAILS

The EPA mandates that the two mos t 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 resul t 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 m ind when selling an aftermarket converter that all trouble code conditions

contributing to the failure, such as an engine m isfire, mus t be corrected when the converter is ins talled.

In addition, all technical service bulletins (TSBs) is sued by the auto manufacturer pertaining to catalyst

failures must be reviewed for required reprogramming or recalibration updates. Because a vehicles

PCM is programmed to monitor converters designed for specific applications, never try to subs titute

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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