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Emission System
DIAGNOSING THREE-WAY CONVERTERS
Edited from an article by Gary Goms, ImportCar Magazine
The essence of remaining competitive in the import service industry is to work at the leading edge of modern vehicle technology. Just a few years ago, for example, most of us replaced a catalytic converter only when it disintegrated or when it wouldn’t pass a local emissions test. Today, we’re finding ourselves replacing converters when a customer brings in his vehicle with an illuminated malfunction indicator light (MIL) and a set of diagnostic trouble codes that may indicate when the efficiency of the catalytic converter has fallen below federal and state standards.
To meet pre-1996 emissions standards, "two-way" catalytic converters were designed to convert two regulated gases, hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2) and water (H2O). To accomplish this, two-way converters require a slightly richer air/fuel ratio to help limit nitrogen oxides (NOx) and a secondary air injection system at the converter to help oxidize HC and CO.
To meet post-1996 emissions standards, the modern "three-way converter" (TWC) converts three regulated exhaust emissions - carbon monoxide, hydrocarbons and nitrogen oxides - into harmless gases. Unlike OBD I emissions control systems, the OBD II powertrain control module (PCM) is designed to automatically test the efficiency of the TWC and other emissions-related components during specific driving conditions. In conventional systems equipped with titania-based oxygen sensors, the PCM tests the TWC by comparing the voltage-switching rate of a pre-catalytic or upstream titania-based oxygen sensor with that of a post-catalytic or downstream titania-based oxygen sensor.
While each vehicle application may follow differing operating strategies, the PCM generally alternates the air/fuel mixture from rich to lean, which means that the oxygen sensor voltage is switching from about 0.2 to 0.8-volts. Obviously, before the PCM can test the efficiency of the TWC, it must test each oxygen sensor for functions such as response time, switching rate and voltage range. If, for example, the voltage range is biased high or low, or if the oxygen sensor has degraded in sensitivity, the PCM will store at least one of many different oxygen sensor-related diagnostic trouble codes (DTCs). Assuming that the oxygen sensors are in good condition, the diagnostic "cat monitor" in the PCM then begins testing the TWC by comparing the switching rates of the pre-cat and post-cat signals.
THE DREADED P0420
To illustrate how the "cat monitor" computes data, I’ve included three scan tool graphs of a failing catalytic converter from a vehicle that has, in fact, just stored a DTC P0420, which indicates that the catalyst system efficiency is below threshold on bank one. After a thorough road test, the upstream bank 1, sensor 1 (B1 S1) graph shows a high switching rate. Its companion downstream sensor, the B1 S2 sensor, indicates a steady 629 millivolts (mV) output, which is as it should be. On the other hand, while the B2 S1 sensor is showing a normal 0.2 to 0.8 volt switching rate , its companion B2 S2 sensor is practically flat-lined at 56 mV. Since the converter is in its earliest stages of failure, a 2,000-rpm throttle snap shows some response, and B2 duplicates the B1 voltage graph up to the point that the throttle snaps closed. The B2 S2 voltage then goes back to about 60 mv. After the engine idles about 10 minutes and lets the converter cool off, the B1 S2 switching rate begins to increase, while the B2 S2 rate continues as before.
BASIC OPERATING PRINCIPLES
To delve a bit deeper into modern TWC operating principles, the main difference between a pre-1996 OBD I converter and a post-1996 OBD II TWC, is the TWC’s ability to store oxygen in order to help break down nitrogen oxide pollutants.
To do this, the TWC combines an oxidation function to convert HCs and CO and a reduction function to convert NOx. The reduction function adds cerium to the catalyst in order to reduce NOx compounds into nitrogen and hydrogen. Unlike OBD I converters that required a slightly richer air/fuel mixture to limit NOx emissions, the OBD II converter operates at a chemically correct stoichiometric 14.7: 1 air/fuel ratio.
The TWC’s oxidation catalyst includes small quantities of precious metals like platinum, palladium and rhodium that will break down HC and O2 to form CO2 and H2O; and CO and O2 into CO2. The reduction catalyst includes cerium, which stores and releases oxygen. This oxygen storage capability helps break down nitric oxide and hydrogen (H) into nitrogen (N) and water vapor.
Both of these chemical reactions are the result of a "catalyst light-off" that begins when hot exhaust gases bring the catalyst temperature up to a minimum operating temperature of 475-575° F.
Of course, if the converter begins oxidizing excessive amounts of raw gasoline and oxygen from a misfiring cylinder, it will exceed its maximum operating temperature of 1,400-1,800° F. In this case, the catalyst will begin to melt and create debris that clogs the converter and muffler outlets.
A DIAGNOSTIC OVERVIEW
Before proceeding with a converter diagnosis, it’s important to remember that auto manufacturers are required to warranty the PCM and the catalytic converter for eight years and 80,000 miles. Converter failures under this time and mileage limit must be referred back to the OEM dealership.
Next, it’s also important to remember that, since each engine has its own specific emissions characteristics, three-way converters are very application-specific and usually are built as an integral part of the vehicle’s exhaust system. Consequently, aside from OEM sources, only a few aftermarket sources exist for replacement catalytic converters.
As of the 2000 model year, there are 19 generic OBD II trouble codes related to the catalytic converter. Of these DTCs, the P0420 Bank 1 and P0430 Bank 2 failures are probably the most common. If non-related codes are retrieved along with TWC-related codes, always consult a shop manual for the correct diagnostic procedure. In most cases, all other DTCs should be corrected before proceeding with the diagnosis of a TWC code.
There are also a number of manufacturer-specific, "P1" codes that relate to various types of catalytic converter failures that can be retrieved by aftermarket scan tools. Also, an occasional "false" DTC may be stored because the threshold criteria of the software have been written to an excessively high standard. Because the software used to program each PCM is very application-specific, many nameplate specialists have developed very specific methods of separating false DTCs for actual catalytic converter failures. These methods might include analyzing the reaction time in milliseconds to the infusion of propane gas, and they might also include simply clearing the DTC and "reflashing" the PCM.
Since most causes of failure include poisoning by ethyl lead, such as that contained in racing or aviation gasoline, phosphorus from fuel and oil additives, zinc from oil additives, and silicon from dirt-contaminated fuel or from non-recommended RTV silicone gasket compounds, it’s always important to determine the root cause of failure before, not after, replacing an expensive catalytic converter.
Consequently, it’s clear that understanding how to effectively use an aftermarket or OEM scan tool to diagnose TWC failures is a must in today’s import service market. When selecting a scan tool, nameplate specialists would be well advised to look into purchasing an OEM-sourced, dedicated scan tool that will provide the maximum amount of converter-related data for specific nameplate applications.
Multi-nameplate shops, on the other hand, may find it more feasible to invest in an aftermarket scan tool that provides most of the basic functions needed to diagnose failing TWCs. In the accompanying photos, for example, my scan tool illustrated the problem by graphing all four oxygen sensors. For the most part, graphing is a very handy feature for diagnosing or confirming any TWC failure. Above all, it’s important for each technician to get current training in scan tool trouble code and data interpretation in order to prevent making expensive diagnostic mistakes.
SELLING PREVENTIVE MAINTENANCE
Obviously, an import shop can’t legally sell a catalytic converter until it has become physically damaged or until it fails an emissions inspection. Nevertheless, any repair information system has application-specific maintenance schedules that recommend preventive services that will extend catalytic converter life, including such "minor" services as oil, filter, lube and coolant changes. An information system may also recommend spark plug, spark plug wire and oxygen sensor replacements, which are part of keeping the modern, three-way catalytic converter alive and healthy.
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