Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems

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POSITIVE CRANKCASE VENTILATION AND SECONDARY AIR-INJECTION SYSTEMS 341

destroying the air filter and greatly reducing engine

power.)



Drive belt on an engine-driven pump, for wear and proper

tension (If the belt is worn or damaged, check that the

AIR pump rotates.)

FOUR-GAS EXHAUST ANALYSIS An SAI system can

be easily tested using an exhaust gas analyzer and the follow-

ing steps:

1. Start the engine and allow it to run until normal operating

temperature is achieved.

2. Connect the analyzer probe to the tailpipe and observe

the exhaust readings for hydrocarbons (HC) and carbon

monoxide (CO).

CHART 27–1

Typical SAI system operation showing location of airflow from

the pump.

Engine Operation

Normal Operation

of a Typical SAI System

Cold engine (open-loop

operation)

Air is diverted to the exhaust

manifold(s) or cylinder head

Warm engine (closed-loop

operation)

Air is diverted to the catalytic

converter

Deceleration Air is diverted to the air cleaner

assembly

Wide-open throttle Air is diverted to the air cleaner

assembly

SAI-RELATED DIAGNOSTIC

TROUBLE CODE

Diagnostic

Trouble Code

Description

Possible Causes

P0410 SAI solenoid

circuit fault

• Defective SAI

solenoid

• Loose or cor-

roded electrical

connections

• Loose, missing,

or defective

rubber hose(s)

3. Using the appropriate pinch-off pliers, shut off the airflow

from the SAI system. Observe the HC and CO readings. If

the SAI system is working correctly, the HC and CO levels

should increase when the SAI system is shut off.

4. Record the O

reading with the SAI system still inoperative.

Unclamp the pliers and watch the O

readings. If the sys-

tem is functioning correctly, the O

level should increase

by 1% to 4%.

4. PCV tests include the rattle test, card test, snap-back test,

and crankcase vacuum test.

5. The AIR system forces air at low pressure into the exhaust

to reduce CO and HC exhaust emissions.

6. Exhaust check valves are used between the AIR pump and

the exhaust manifold to prevent exhaust gases from flow-

ing into and damaging the AIR pump and valves.

1. Positive crankcase ventilation (PCV) systems use a valve

or a fixed orifice to control the crankcase vapors from the

crankcase back into the intake system.

2. A PCV valve regulates the flow of crankcase vapors

depending on engine vacuum and seals the crankcase

vent in the event of a backfire.

3. As much as 30% of the air needed by the engine at idle

speed flows through the PCV system.

SUMMARY

3. What does the abbreviation PCV mean?

4. What does the abbreviation AIR mean?

1. What exhaust emissions do the PCV valve and the SAI

system control?

2. How does a PCV valve work?

REVIEW QUESTIONS

342 CHAPTER 27

7. Two technicians are discussing exhaust check valves used

in SAI systems. Technician A says that they are used to

prevent the output from the SAI pump from entering the

intake manifold. Technician B says the check valves are

used to keep the exhaust from entering the AIR pump.

Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

8. Where is the output of the AIR pump directed when the

engine is cold?

a. Exhaust manifold

b. Catalytic converter

c. Air cleaner assembly

d. To the atmosphere

9. The switching valves on the AIR pump have failed several

times. Technician A says that a defective exhaust check

valve could be the cause. Technician B says that a leaking

exhaust system at the muffler could be the cause. Which

technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

10. When checking for the proper operation of the AIR sys-

tem using an exhaust gas analyzer, how much should the

oxygen (O

) levels increase when the pump is allowed to

function?

a. 1% to 4%

b. 5% to 10%

c. 10% to 20%

d. Up to 30%

1. The PCV system controls which exhaust emission(s)?

a. HC

b. CO

c. NO

d. Both HC and CO

2. How much of the air needed by the engine flows through

the PCV system when the engine is at idle speed?

a. 1% to 3%

b. 5% to 10%

c. 10% to 20%

d. Up to 30%

3. Technician A says that if the PCV valve was defective or

clogged, the engine could idle rough. Technician B says

that the engine may stall. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

4. Technician A says that if a PCV valve rattles, then it is okay

and does not need to be replaced. Technician B says that

if a PCV valve does not rattle, it should be replaced. Which

technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

5. Technician A says that the PCV system should create a

slight pressure in the crankcase at idle. Technician B says

that the PCV system should create a slight vacuum in the

crankcase at 2,500 RPM. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

6. The SAI system is used to reduce which exhaust

emission(s)?

a. HC

b. CO

c. NO

d. Both HC and CO

CHAPTER QUIZ

CATALYTIC CONVERTERS 343

chapter

CATALYTIC CONVERTERS

OBJECTIVES: After studying Chapter 28 , the reader should be able to: • Prepare for ASE Engine Performance (A8)

certification test content area “D” (Emission Control Systems). • Describe the purpose and function of the catalytic converter.

• Discuss how the catalytic converter is tested under OBD-II regulations. • Explain the method for diagnosing and testing the

catalytic converter.

KEY TERMS: Catalysts 343 • Catalytic converter 343 • Cerium 344 • Light-off temperature 344 • Light-off converter

(LOC) 345 • Oxygen storage capacity (OSC) 345 • Palladium 344 • Platinum 344 • Preconverter 345 • Pup (mini)

converter 345 • Rhodium 344 • Tap test 346 • Three-way converter (TWC) 344 • Washcoat 343

PURPOSE AND FUNCTION A catalytic converter is an

aftertreatment device used to reduce exhaust emissions out-

side of the engine. The catalytic converter uses a catalyst:



A catalyst is a chemical that helps start a chemical

reaction but does not enter into the chemical reaction.



The catalyst materials on the surface of the material

inside the converter help create a chemical reaction.



The chemical reaction changes harmful exhaust

emissions into nonharmful exhaust emissions.



The converter therefore converts harmful exhaust gases

into water vapor (H

O) and carbon dioxide (CO

This device is installed in the exhaust system between

the exhaust manifold and the muffler and usually is positioned

beneath the passenger compartment. The location of the con-

verter is important since as much of the exhaust heat as pos-

sible must be retained for effective operation. The nearer it is to

the engine, the better.

 SEE FIGURE 28–1 .

CATALYTIC CONVERTER CONSTRUCTION Most cat-

alytic converters are constructed of a ceramic material in a

honeycomb shape with square openings for the exhaust gases:



There are approximately 400 openings per square

inch (62 per sq. cm), and the wall thickness is about

0.006inch (1.5 mm).



The substrate is then coated with a porous aluminum

material called the washcoat, which makes the surface

rough.



The catalytic materials are then applied on top of the

washcoat. The substrate is contained within a round or

oval shell made by welding together two stamped pieces

of aluminum or stainless steel.

 SEE FIGURE 28–2 .

The ceramic substrate in monolithic converters is not

restrictive; however, the converter can be physically broken if

exposed to shock or severe jolts. Monolithic converters can be

serviced only as a unit.

An exhaust pipe is connected to the manifold or header to

carry gases through a catalytic converter and then to the muffler

or silencer. V-type engines can use dual converters or route the

exhaust into one catalytic converter by using a Y-exhaust pipe.

CATALYTIC CONVERTERS

FIGURE 28–1 Most catalytic converters are located as close

to the exhaust manifold as possible, as seen in this display of

a Chevrolet Corvette.

344 CHAPTER 28

Since the early 1990s, many converters also contain

cerium, an element that can store oxygen. The purpose of the

cerium is to provide oxygen to the oxidation bed of the con-

verter when the exhaust is rich and lacks enough oxygen for

proper oxidation. When the exhaust is lean, the cerium absorbs

the extra oxygen. For the most efficient operation, the converter

should have a 14.7:1 air-fuel ratio but can use a mixture that

varies slightly:



A rich exhaust is required for reduction—stripping the

oxygen (O

) from the nitrogen in NO



A lean exhaust is required to provide the oxygen neces-

sary to oxidize HC and CO (combining oxygen with HC

and CO to form H

O and CO

If the catalytic converter is not functioning correctly, en-

sure that the air-fuel mixture being supplied to the engine is

correct and that the ignition system is free of defects.

CONVERTER LIGHT-OFF TEMPERATURE The cata-

lytic converter does not work when cold, so it must be heated

to its light-off temperature of close to 500°F (260°C) before it

starts working at 50% effectiveness. When fully effective, the

converter reaches a temperature range of 900°F to 1,600°F

(482°C to 871°C). In spite of the intense heat, however, cata-

lytic reactions do not generate a flame associated with a simple

burning reaction. Because of the extreme heat (almost as hot

as combustion chamber temperatures), a converter remains hot

long after the engine is shut off. Most vehicles use a series of

heat shields to protect the passenger compartment and other

parts of the chassis from excessive heat. Vehicles have been

known to start fires because of the hot converter causing tall

grass or dry leaves beneath the just-parked vehicle to ignite,

especially if the engine is idling. This is most likely to occur if the

heat shields have been removed from the converter.

CONVERTER USAGE A catalytic converter must be

located as close as possible to the exhaust manifold to work

effectively. The farther back the converter is positioned in the

exhaust system, the more the exhaust gases cool before they

CATALYTIC CONVERTER OPERATION The converter

substrate contains small amounts of rhodium, palladium, and

platinum. These elements act as catalysts. As mentioned, a

catalyst is an element that starts a chemical reaction with-

out becoming a part of, or being consumed in, the process.

In a three-way (catalytic) converter (TWC) all three exhaust

emissions (NO

, HC, and CO) are converted to carbon dioxide

(CO

) and water (H

O). As the exhaust gas passes through the

catalyst, oxides of nitrogen (NO

) are chemically reduced (that

is, nitrogen and oxygen are separated) in the first section of

the catalytic converter. In the second section of the catalytic

converter, most of the hydrocarbons and carbon monoxide

remaining in the exhaust gas are oxidized to form harmless car-

bon dioxide (CO

) and water vapor (H

O).  SEE FIGURE 28–3 .

MONOLITHIC SUBSTRATE

WIRE MESH

SLEEVE

CONVERTER

HOUSING

FIGURE 28–2 A typical catalytic converter with a monolithic

substrate.

FIGURE 28–3 The three-way catalytic converter first separates the NO

into nitrogen and oxygen and then converts the HC and

CO into harmless water (H

O) and carbon dioxide (CO

). The nitrogen (N) passes through the converter and exits the tailpipe and

enters the atmosphere which is about 78% nitrogen.

CATALYTIC CONVERTERS 345

These factors are the enable criteria that must be achieved

before the OBD-II catalyst monitor will run.

reach the converter. Since positioning in the exhaust system

affects the oxidation process, vehicle manufacturers that use

only an oxidation converter generally locate it underneath the

front of the passenger compartment.

Some vehicles have used a small, quick heating oxidation

converter called a preconverter or a pup (mini) converter that

connects directly to the exhaust manifold outlet. These have

a small catalyst surface area close to the engine that heats

up rapidly to start the oxidation process more quickly during

cold engine warm-up. For this reason, they were often called

light-off converters (LOCs). The larger main converter, under

the passenger compartment, completes the oxidation reaction

started in the LOC.

Most older vehicles used a catalytic converter that had the

reduction section first to separate the oxygen from the nitrogen

in NOx. The oxygen released during this action help provide

extra oxygen to help oxidize the HC and CO into harmless water

O) and carbon dioxide (CO

). However, since 2004, emission

standards for oxides of nitrogen are stricter, and therefore a

larger reduction section is often needed. Therefore, the reduc-

tion section is now often after the oxidation section, which is

the opposite of the way it was in converters for older models.

 SEE FIGURE 28–4 .

OBD-II CATALYTIC CONVERTER PERFORMANCE

The PCM determines if the catalytic converter is ready for

testing based on the following conditions, which may vary by

vehicle make, model, and year:



Closed-loop status achieved



IAT sensor temperature higher than 32°F (0°C)



ECT sensor temperature higher than 165°F (18°C)



MAF sensor input from 15 to 32 g/sec



Engine load less than 65% and steady



Engine speed less than 4,000 RPM



All of the above conditions met for at least four minutes

OXIDATION BRICK

(HC AND CO)

FLOW

EXHAUST FLOW

REDUCTION BRICK

(NO

)

FIGURE 28–4 The small oxidation section of the converter

helps build heat for the reduction section to reduce NO

emissions in the rear brick on most newer vehicles.

UPSTREAM

OXYGEN SENSOR

CATALYTIC CONVERTER

RAPIDLY

SWITCHING

SIGNAL

SLOWLY

SWITCHING

SIGNAL OR

STRAIGHT LINE

DOWNSTREAM

OXYGEN SENSOR

FIGURE 28–5 The OBD-II catalytic converter monitor

compares the signals of upstream and downstream oxygen

sensors to determine converter efficiency.

OBD-II CATALYTIC

CONVERTER PERFORMANCE

With OBD-II-equipped vehicles, catalytic converter perfor-

mance is monitored by a heated oxygen sensor (HO

S) both

before and after the converter.

 SEE FIGURE 28–5 .

The converters used on these vehicles have oxygen

storage capacity (OSC), due mostly to the cerium coating in

the catalyst rather than the precious metals used. When the

three-way converter (TWC) is operating as it should, the post-

converter HO

S is far less active than the preconverter sensor.

The converter stores, then releases, the oxygen during normal

reduction and oxidation of the exhaust gases, smoothing out

the variations in oxygen being released.

Where a cycling sensor voltage output is expected before

the converter, because of the converter action, the postcon-

verter HO

S should read a steady signal with little fluctuation.

 SEE FIGURE 28–6 .

NOTE: Because of more demanding exhaust emission

standards for oxides of nitrogen (NO

) starting in 2004,

the reduction part of the converter is now usually located

downstream from the oxidation section. This is opposite

of the way an older style converter was constructed. With

better fuel injection systems and ignition systems, the

amount of CO and HC exhaust emissions is lower than

in the past. By using the oxidation section first creates

heat to help the reduction section better control oxides of

nitrogen emissions. The rear “brick” or reduction section

is also larger than the oxidation section.

346 CHAPTER 28

CONVERTER-DAMAGING CONDITIONS Since con-

verters have no moving parts, they require no periodic ser-

vice. Under federal law, catalyst effectiveness is warranted for

80,000 miles or eight years.

The three main causes of premature converter failure are

the following:



Contamination. Substances that can destroy the con-

verter include exhaust that contains excess engine oil,

antifreeze, sulfur (from poor fuel), and various other

chemical substances.



Excessive temperatures. Although a converter oper-

ates at high temperatures, it can be destroyed by exces-

sive temperatures. This most often occurs either when

too much unburned fuel enters the converter, or with

excessively lean mixtures. Excessive temperatures may

be caused by long idling periods on some vehicles since

more heat develops at those times than when driving at

normal highway speeds. Severe high temperatures can

cause the converter to melt down, leading to the internal

parts breaking apart and either clogging the converter or

moving downstream to plug the muffler. In either case, the

restricted exhaust flow severely reduces engine power.



Improper air-fuel mixtures. Rich mixtures or raw fuel

in the exhaust can be caused by engine misfiring or an

excessively rich air-fuel mixture resulting from a defec-

tive coolant temp sensor or defective fuel injectors. Lean

mixtures are commonly caused by intake manifold leaks.

When either of these circumstances occurs, the converter

can become a catalytic furnace, causing the previously

described damage. For most efficient catalytic converter

operation, the air-fuel mixture should be near 14.7:1.

 SEE FIGURE 28–7 .

To avoid excessive catalyst temperatures and the possibility

of fuel vapors reaching the converter, observe the following rules:

1. Do not use fuel additives or cleaners that are not converter

safe.

2. Do not crank an engine for more than 40 seconds when it

is flooded or misfiring.

3. Do not turn off the ignition switch when the vehicle is in motion.

PRE-

CAT

POST

CAT

FIGURE 28–6 The waveform of a downstream O2S sensor

from a properly functioning converter shows little, if any, activity.

100

13:1 14:1 15:1 16:1

CONVERSION

EFFICIENCY

AIR-FUEL RATIO

FIGURE 28–7 The highest catalytic converter efficiency

occurs when the air-fuel mixture is about 14.7:1.

Can a Catalytic Converter Be Defective

withoutBeing Clogged?

Yes. Catalytic converters can fail by being chemically

damaged or poisoned without being mechanically

clogged. Therefore, the catalytic converter should

be tested not only for physical damage (clogging) by

performing a back pressure or vacuum test and a

rattle test but also for temperature rise, usually with a

pyrometer or propane test, to check the efficiency of

the converter.

FREQUENTLY ASKED QUESTION

DIAGNOSING

CATALYTIC CONVERTERS

TAP TEST The simple tap test involves tapping (not pound-

ing) on the catalytic converter using a rubber mallet. If the

substrate inside the converter is broken, the converter will rattle

when hit. If the converter rattles, a replacement converter is

required.

 SEE FIGURE 28–8 .

TESTING BACK PRESSURE WITH A PRESSURE

GAUGE Exhaust system back pressure can be measured

directly by installing a pressure gauge in an exhaust opening.

This can be accomplished in one of the following ways:

1. To test an oxygen sensor, remove the inside of an old, dis-

carded oxygen sensor and thread in an adapter to convert

it to a vacuum or pressure gauge.

4. Do not disconnect a spark plug wire for more than 30 seconds.

5. Repair engine problems such as dieseling, misfiring, or

stumbling as soon as possible.

CATALYTIC CONVERTERS 347

STEP 1 Attach a vacuum gauge to an intake manifold vacuum

source.

STEP 2 Start the engine. Record the engine manifold vacuum

reading. The engine vacuum should read 17 to 21 in. Hg

when the engine is at idle speed.

STEP 3 Increase the engine speed to 2,500 RPM and hold that

speed for 60 seconds while looking at the vacuum

gauge.

Results



If the vacuum reading is equal to or higher than the

vacuum reading when the engine was at idle speed, the

exhaust system is not restricted.

NOTE: An adapter can be easily made by inserting a

metal tube or pipe into an old oxygen sensor hous-

ing. A short section of brake line works great. The

pipe can be brazed to the oxygen sensor housing, or

it can be glued with epoxy. An 18-mm compression

gauge adapter can also be adapted to fit into the

oxygen sensor opening.

 SEE FIGURE 28–9 .

2. To test the exhaust back pressure at the exhaust gas recir-

culation (EGR) valve, remove the EGR valve and fabricate

a plate equipped with a fitting for a pressure gauge.

3. To test at the secondary air-injection (SAI) check valve, remove

the check valve from the exhaust tubes leading to the exhaust

manifold. Use a rubber cone with a tube inside to seal against

the exhaust tube. Connect the tube to a pressure gauge.

At idle, the maximum back pressure should be less than

1.5 PSI (10 kPa), and it should be less than 2.5 PSI (15 kPa)

at 2,500 RPM. Pressure readings higher than these indicate

that the exhaust system is restricted, and further testing will

be needed to determine the location of the restriction.

 SEE

FIGURE 28–10 .

TESTING FOR BACK PRESSURE USING A VACUUM

GAUGE An exhaust restriction can be tested indirectly by

checking the intake manifold vacuum with the engine operat-

ing at a fast idle speed (about 2,500 RPM). If the exhaust is

restricted, some exhaust can pass, and the effect may not be

noticeable when the engine is at idle speed. However, when the

engine is operating at a higher speed, the exhaust gases can

build up behind the restriction and eventually will not be able to

leave the combustion chamber. When some of the exhaust is

left behind at the end of the exhaust stroke, the resulting pres-

sure in the combustion chamber reduces engine vacuum. To

test for an exhaust restriction using a vacuum gauge, perform

the following steps:

FIGURE 28–8 A catalytic converter that rattles when tapped

was removed, and the substrate, or what was left of it, fell out.

This converter has to be replaced and the root cause of why it

failed found and corrected.

EXHAUST

MANIFOLD

STEEL

BRAKE

LINE

OXYGEN SENSOR

HOUSING

BRAZING

ADAPTER

FIGURE 28–9 A back pressure tool can be made by using an

oxygen sensor housing and epoxy or braze to hold the tube to

the housing.

FIGURE 28–10 This partially melted catalytic converter

tested okay at idle but had excessive back pressure at idle

speeds.

348 CHAPTER 28



If O

is greater than zero, check the CO level.



If CO is greater than zero, the converter is not

functioning correctly.



Snap-throttle test. With the engine warm and in closed

loop, snap the throttle to wide-open throttle (WOT) in

park or neutral and observe the oxygen reading:



The O

reading should not exceed 1.2%; if it does, the

converter is not working.



If the O

rises to 1.2%, the converter may have low

efficiency.



If the O

remains below 1.2%, then the converter

is okay.



If the vacuum reading is lower than the vacuum reading

when the engine was at idle speed, then the exhaust is

restricted. Further testing will be needed to determine the

location of the restriction.

TESTING A CATALYTIC CONVERTER FOR TEMPER-

ATURE RISE A properly working catalytic converter should

be able to reduce NO

exhaust emissions into nitrogen (N)

and oxygen (O

) and oxidize unburned hydrocarbon (HC) and

carbon monoxide (CO) into harmless carbon dioxide (CO

)

and water vapor (H

O). During these chemical processes,

the catalytic converter should increase in temperature at

least 10% if the converter is working properly. To test the

converter, operate the engine at 2,500 RPM for at least two

minutes to fully warm the converter. Measure the inlet and

the outlet temperatures using an infrared pyrometer as shown

 SEE FIGURE 28–11 .

NOTE: If the engine is extremely efficient, the converter

may not have any excessive unburned hydrocarbons or

carbon monoxide to convert! In this case, a spark plug

wire could be grounded out using a vacuum hose and a

test light to create some unburned hydrocarbon in the

exhaust. Do not ground out a cylinder for longer than

10 seconds, or the excessive amount of unburned hydro-

carbon could overheat and damage the converter.

CATALYTIC CONVERTER EFFICIENCY TESTS The

efficiency of a catalytic converter can be determined using an

exhaust gas analyzer:



Oxygen level test. With the engine warm and in closed

loop, check the oxygen (O

) and carbon monoxide (CO)

levels. A good converter should be able to oxide the

extra hydrocarbons caused by the rapid acceleration:



If O

is zero, go to the snap-throttle test.

FIGURE 28–11 The temperature of the outlet should be at

least 10% hotter than the temperature of the inlet. If a con-

verter is not working, the inlet temperature will be hotter than

the outlet temperature.

Aftermarket Catalytic Converters

Some replacement aftermarket (nonfactory) catalytic

converters do not contain the same amount of ce-

rium as the original part. Cerium is the element that

is used in catalytic converters to store oxygen. As a

result of the lack of cerium, the correlation between

the oxygen storage and the conversion efficiency

may be affected enough to set a false diagnostic

trouble code (P0422).

NOTE: If an aftermarket converter is being in-

stalled, be sure that the distance between the

rear of the catalyst block is the same distance

from the rear oxygen sensor as the factory con-

verter to be ensured of proper operation. Al-

ways follow the instructions that come with the

replacement converter.

 SEE FIGURE 28–12 .

TECH TIP

CATALYTIC CONVERTER

REPLACEMENT GUIDELINES

Because a catalytic converter is a major exhaust gas emission

control device, the U.S.Environmental Protection Agency (EPA)

has strict guidelines for its replacement, including the following:



If a converter is replaced on a vehicle with less than

80,000 miles or eight years, depending on the year of the

vehicle, an original equipment catalytic converter must be

used as a replacement.



The replacement converter must be of the same design

as the original. If the original had an AIR pump fitting, so

must the replacement.



The old converter must be kept for possible inspection by

the authorities for 60 days.

CATALYTIC CONVERTERS 349



A form must be completed and signed by both the vehi-

cle owner and a representative from the service facility.

This form must state the cause of the converter failure

and must remain on file for two years.

CATALYTIC CONVERTER–RELATED DIAGNOSTIC

TROUBLE CODE

REPLACEMENT CONVERTER

NOX

REAR (POST)

SENSOR

KEEP THIS DISTANCE

FRONT

EXHAUST PIPE

NOX

FIGURE 28–12 Whenever replacing a catalytic converter with a universal unit, first measure the distance between the rear brick

and the center of the rear oxygen sensor. Be sure that the replacement unit is installed to the same dimension.

Catalytic Converters Are Murdered

Catalytic converters start a chemical reaction but

do not enter into the chemical reaction. Therefore,

catalytic converters neither wear out nor die of old

age. If a catalytic converter is found to be defective

(nonfunctioning or clogged), look for the root cause.

Remember this:

“Catalytic converters do not commit suicide—

they’re murdered.”

Items that should be checked when a defective

catalytic converter is discovered include all com-

ponents of the ignition and fuel systems. Excessive

unburned fuel can cause the catalytic converter

to overheat and fail. The oxygen sensor must be

working and fluctuating from 0.5 to 5 Hz (times per

second) to provide the necessary air-fuel mixture

variations for maximum catalytic converter efficiency.

TECH TIP

Diagnostic

Trouble Code

Description

Possible Causes

P0422 Catalytic converter

efficiency failure

• Engine mechanical

fault

• Exhaust leaks

• Fuel contaminants,

such as engine oil,

coolant, or sulfur

350 CHAPTER 28

CATALYTIC CONVERTER OPERATION

Carbon monoxide leaves the engine through the exhaust

valve on the exhaust stroke.

The CO molecule is starting to enter the converter.

A CO molecule is ready to enter a cell. The number of

cells ranges from 300 to 900 per sq. in. The substrate is

cordierite (Mg, AL, Si) or foil-backed metal with a 0.002-

to 0.006-inch (0.05- to 0.15-mm)-thick wall.

The CO molecule enters a cell. The substrate is coated

with porous aluminum (AL

) called the washcoat. The

catalytic material is sprayed onto the washcoat.

The CO molecule enters a micropore, which has been

created in the porous washcoat.

The CO molecule enters a smaller micropore.

CELL

CO MOLECULE

SUBSTRATE

WASHCOAT

MACROPORE

WASHCOAT

CATALYST

MATERIAL