Catalytic Converter 101: A Basic Understanding Of How Catalytic Converters Operate – UnderhoodService

Catalytic Converter 101: A Basic Understanding Of How Catalytic Converters Operate

This honeycomb substrate has broken up and clogged the catalytic converter outlet.

When servicing catalytic converters, it’s important to have a basic understanding of how they operate. 

To begin, an internal combustion engine creates heat energy by igniting a mixture of gasoline and atmospheric oxygen with a high-voltage spark. Unfortunately, a residual quantity of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOX) are left over after combustion. Consequently, a catalytic converter uses precious metals like platinum as a catalyst to convert harmful pollutants like HC, CO and NOX into harmless gases like oxygen (O2), carbon dioxide (CO2) and water (H2O). Since a catalyst will accelerate a chemical reaction without being consumed by the process itself, the catalytic converter will theoretically last forever. In reality, catalytic converters eventually succumb to heat stress and contaminants from coolant and engine oil additives.

Converter types
“Direct-fit” catalytic converters are of three basic types: two-way, three-way and three-way plus oxidation converters. Two-way “oxidation” converters are used up to 1980 and are designed to eliminate hydrocarbons (HC) and carbon monoxide (CO). Three-way converters are designed to eliminate nitrogen oxides (NOX) as well as HC and CO. A Three-way plus or “dual-bed” converter allows atmospheric air to be injected between the three-way catalyst and an extra oxidation bed to further clean the engine exhaust. The precious metals used as catalysts are distributed in molecular form over a rough-surfaced ceramic wash coat covering the metallic honeycomb substrate. The effectiveness of the converter is basically determined by the amount of precious metals present on the honeycomb substrate. Consequently, it’s very important for supplier and technician alike to meet EPA requirements by selecting the correct catalytic converter for the application.

The catalyst monitor
The catalyst monitor or self-test runs only once after the engine is started and driven. Basically, the engine control module (ECM) uses a mathematical algorithm programmed into the ECM software to compare the electrical activity of the upstream oxygen sensor (before the converter) with that of the downstream oxygen sensor. When the electrical activity of the upstream and downstream oxygen sensors doesn’t meet programmed parameters, a diagnostic trouble code (DTC) P0420 and/or P0430 is stored in the ECM’s diagnostic memory and the orange “Check Engine” warning light is illuminated. 

Because the catalyst monitor is mathematically-based, attempting to diagnose a converter by comparing inlet and outlet temperatures or by using an exhaust gas analyzer will not yield valid test results. When a P420/430 diagnostic trouble code (DTC) is present, the professional technician should check for related ECM calibration update bulletins before replacing a catalytic converter. In many cases, reprogramming the PCM might solve a stubborn P420/430 DTC issue.

Causes of catalyst failure
An ignition system misfire is the most common cause of catastrophic converter failure. When a misfire occurs, the catalytic converter overheats because combustion takes place in the converter rather than the engine. When the converter exceeds an operating temperature of about 1,300 degrees F, the converter substrate begins to melt and cause exhaust restriction. Common failures also include a catalyst suffering from an external impact or being contaminated by coolant from leaking cylinder head gaskets. 

When to sell catalytic converters 
The auto manufacturer’s original equipment (OE) catalytic converters are initially covered by an EPA-mandated 8-year, 80,000-mile warranty. A new aftermarket catalytic converter can be installed only if the original is missing, fails a state or local emissions inspection program or is plugged, leaking, or has sustained physical damage. Most vehicles require catalytic converter replacements when an illuminated “Check Engine” warning light indicates a converter failure or if the engine itself has failed an exhaust emissions test. To pass an exhaust emissions test, the converter must meet the exact configuration and capacity as specified by the original equipment (OE) manufacturer. Due to adopting more stringent emissions standards, many other states are now requiring catalytic converters that meet California specifications. When selling any catalytic converter, remember to fill out the paperwork as required by state and local agencies.

You May Also Like

How To Diagnose Slow or Sluggish Oxygen Sensors

When oxygen sensors are tested, manufacturers will introduce small amounts of oil to measure sensitivity.

An engine management system is always trying to find the perfect air/fuel ratio. But it is next to impossible to walk the line between too rich or too lean. With every revolution of the crankshaft, small changes in the air, fuel and operating conditions can cause changes to the oxygen content coming out of the exhaust port.

E-15 Ethanol Damage to Fuel Pumps

One of the problems with ethanol is how it reacts to water in the air and in the tank.

Oxygen Sensor Questions Answered

If an O2 sensor is not reading properly or is borderline, it should be replaced regardless of its age or mileage.

Managing (DPF) Diesel Particulate Filter Regeneration Cycles

The DPF is designed to store the soot and ash, to later burn them off during a regeneration cycle.

Ultimate Underhood – Gasoline Particulate Filters

Just like a DPF, a gasoline particulate filter (GPF) traps and stores soot particles in the exhaust stream.

Other Posts
Direct Fit Catalytic Converters Features and Benefits

Four qualities stand out the most when looking at these catalytic converters. Sponsored by AP Emissions.

Catalytic Converter Theft (VIDEO)

In some cases, thieves can remove a catalytic converter in less than one minute. This video is sponsored by AP Emissions.

Catalytic Converter Development Process (VIDEO)

Experts discuss the catalytic converter development process from start to finish. Sponsored by AP Emissions.

EPA vs California Emissions Packages (VIDEO)

What’s the difference between EPA and California emissions packages? This video is sponsored by AP Emissions.