OBJECTIVES

After studying Chapter 29, the reader will be able to:

1. Prepare for ASE Engine Performance (A8) certification test content area “D” (Emission Control Systems).

2. Describe the purpose and function of the catalytic converter.

3. Discuss how the catalytic converter is tested under OBD-II regulations.

4. Explain the method for diagnosing and testing the catalytic converter.

Automotive Fuel and Emissions Control Systems, 2/eBy James D. Halderman and Jim Linder

© 2009 Pearson Higher Education, Inc.Pearson Prentice Hall - Upper Saddle River, NJ 07458

Chapter 29Catalytic Converter

CATALYTIC CONVERTERS are an after-

treatment device used to reduce exhaust emissions

Is between the exhaust manifold & muffler

usually is positioned beneath the passenger compartment.

Types• Dual Bed

• Old school GM

• Ceramic Monolith• Most

common• Two way

• CO & HC• Three way

• NOx, CO & HC

Catalytic Converter Operation

The converter contains small amounts of rhodium, palladium, cerrium and platinum.

These elements act as catalysts. A catalyst is an element that starts a

chemical reaction without becoming a part of, or being consumed in, the process.

In a three-way catalytic converter (TWC), three exhaust emissions (NOX, HC, and CO) are converted to carbon dioxide (CO2) and water (H2O).

The three-way catalytic converter first separates the NOX into nitrogen and

oxygen and then converts the HC and CO into harmless water (H2O) and carbon dioxide

(CO2).

VIDEO

A cutaway of three-way catalytic converter showing the air tube in the center of the reducing and oxidizing section of the converter. Note the small holes in the

tube to distribute air from the AIR pump to the oxidizing rear section of the converter.

Converter Light-Off

The catalytic converter does not work when cold and it must be heated to its light-off temperature of close to 500°F (260°C) before it starts working at 50% effectiveness.

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 gases cool before they reach the converter.

CONVERTER WARM-UP TEST 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 4000 RPM All of the above conditions met for at least

four minutes

OBD-II CATALYTIC CONVERTER PERFORMANCE With OBD-II equipped

vehicles, catalytic converter performance is monitored by a heated oxygen (HO2S) sensor both before and after the converter.

The OBD-II catalytic converter monitor compares the signals of the upstream and downstream O2Ss to determine converter efficiency.

The waveform of an O2S downstream from a properly functioning converter shows little, if any,

activity.

CONVERTER DAMAGING CONDITIONS The highest catalytic

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

The three main causes of premature converter failure are as follows: Contamination. Excessive

temperatures. Improper air-fuel

mixtures.

THE TAP TEST The simple tap test

involves tapping (not pounding) 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.

Symptoms?

TESTING BACK PRESSUREWITH A VACUUM GAUGE

A vacuum gauge can be used to measure manifold vacuum at a high idle (2000 to 2500 RPM).

If the exhaust system is restricted, pressure increases in the exhaust system.

This pressure is called back pressure. Manifold vacuum will drop gradually if

the engine is kept at a constant speed if the exhaust is restricted.

BACK PRESSURE READINGS

Normal @ idle 1.5 psi most vehicles Some to as much as 2.75

@ 2000 rpm Normal 3 psi (some a bit more) & steady Restricted will build pressure

Also IM vacuum will drop as BP builds @ 4000 rpm

4 to 8 psi normal most vehicles

TESTING PORTS FOR EXHAUST BACK PRESSURE

At front oxygen sensor

At exhaust gas recirculation (EGR) valve

At air injection reaction (AIR) check valve

HIGH BACK PRESSURE

A partially melted catalytic converter that tested okay at idle but had excessive back pressure at

idle speeds.

TESTING A CATALYTIC CONVERTER FOR TEMPERATURE RISE

A properly working catalytic converter should be able to reduce NOX exhaust emissions into nitrogen (N) and oxygen (O2) and oxidize unburned hydrocarbon (HC) and carbon monoxide (CO) into harmless carbon dioxide (CO2) and water vapor (H2O).

TESTING A CATALYTIC CONVERTER FOR TEMPERATURE RISE •The temperature of the

outlet should be at least 10% hotter than the temperature of the inlet. •450°F.Ten percent of 450° is 45° (45°450°495°). •In other words, the outlet temperature should be at least 495°F for the converter to be considered okay. •If the converter is not working at all, the inlet temperature will be hotter than the outlet temperature.

INTRUSIVE TESTING The efficiency of a catalytic converter can be

determined using an exhaust gas analyzer. Oxygen level test.

If O2 is zero, go to the snap throttle test. If O2 is greater than zero, check the CO level. If CO is greater than zero, the converter is not

functioning correctly. Snap-throttle test.

The O2 reading should not exceed 1.2%; if it does, the converter is not working.

If the O2 rises to 1.2%, the converter may have low efficiency.

If the O2% remains below 1.2%, then the converter is okay.

OBD-II CATALYTIC CONVERTER MONITOR The catalytic converter monitor of OBD-

II uses an upstream and downstream HO2S to test catalytic efficiency. When the engine combusts a lean air-fuel

mixture, higher amounts of oxygen flow through the exhaust into the converter.

The catalyst materials absorb this oxygen for the oxidation process, thereby removing it from the exhaust stream.

CATALYTIC CONVERTERRELATED DIAGNOSTIC TROUBLE CODE