Catalytic Converters Keep It Clean

By Gary Goms

Beginning in 1996, the Environmental Protection Agency (EPA) mandated that a new set of emissions standards known to our industry as On-Board Diagnostics II or "OBD II."
Beginning in 1996, the Environmental Protection Agency (EPA) mandated that a new set of emissions standards known to our industry as On-Board Diagnostics II or “OBD II.” The new 1996 OBD II standard included a mandated on-board diagnostic “catalyst monitor” designed to test the catalytic converter’s efficiency during vehicle operation.  When the converter’s efficiency falls below a predetermined threshold of approximately 95 percent, the PCM stores a P0420 and/or P0430 diagnostic trouble code (DTC) and illuminates the orange “Check Engine” warning light.

Catalytic Converter Construction
In brief, a catalyst is an element that initiates a chemical change without itself undergoing chemical change. Modern “three-way” OBD II catalytic converters contain several different catalysts in a ceramic honeycombed substrate designed to expose the maximum area of catalyst while reducing exhaust restriction to zero.

Three-way converters are expensive because they use precious metals like platinum to oxidize exhaust feed gases like carbon monoxide (CO) and hydrocarbons like raw gasoline (HC) into carbon dioxide (CO2) and water (H2O).  Metals like Rhodium are used to reduce nitrogen oxides (NOX), which are formed under extremes of heat and pressure in the engine’s cylinders, into their component elements of nitrogen (N) and oxygen (O2). The tailpipe gases exiting the converter are a harmless mix of water, carbon dioxide, nitrogen and oxygen.

The Catalyst Monitor
The OBD II catalyst monitor measures the catalyst efficiency by comparing voltage output from oxygen sensors located upstream and downstream from the catalyst. A conventional upstream oxygen sensor rapidly switches between .2 and .8 volts, which indicate normal fuel control. If the catalytic converter is oxidizing and reducing exhaust feed gases efficiently, the downstream oxygen sensor should generate a steady voltage ranging from .5 to .7 volts.

To calculate catalyst efficiency, the PCM compares the upstream switching rate with the downstream switching rate of the two oxygen sensors. Because newer systems equipped with upstream air fuel ratio (AFR) sensors don’t react to exhaust gas oxygen the same way, the PCM uses a slightly different strategy to measure catalyst efficiency. In either case, deterioration of the catalyst is indicated when the upstream and downstream oxygen sensor voltages begin to duplicate each other.

Catalytic Converter Failures
In any case, the catalyst monitoring software contained within the PCM is the final arbiter of the efficiency of any post-1996 catalytic converter.  Since an OBD II catalyst is a wearing part, most catalytic converters generally begin to fail at 120,000 miles or more. In contrast, most premature catalyst failures are caused by misfiring ignition systems, which cause an excessive amount of raw gasoline to be fed into the exhaust gas stream. During a severe ignition misfire, the catalyst temperature exceeds 1,500 degrees F. temperature, which causes the catalyst to melt. In many cases, the melted catalyst reduces engine performance by restricting the flow of exhaust gases from the engine.

In other instances of premature failure, the catalyst is generally coated with chemicals like phosphorous from engine oil or coolant leaking into the exhaust stream. In this case, the engine will perform well, but the catalyst will fail the efficiency test.

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