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
FUMES IN, DAISIES OUT
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.
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).
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
CONVERTING EXHAUST GASES
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
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
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
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
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.
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.
OXYGEN SENSOR DIAGNOSIS
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.
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.
WHEN A CONVERTER FAILS
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.
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).