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Catalytic Converters 101: Powering Eco-Friendly Driving

By R&D
Published on March 20th, 2024

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The “catalytic converter” is an emissions control device creating chemical reactions that convert specific pollutant chemicals in the exhaust to non-polluting chemicals. 

Catalytic Converter

Modern converters oxidize hydrocarbons (HC) to carbon dioxide and water vapor, oxidize carbon monoxide (CO) to carbon dioxide, and reduce nitrogen oxide (NOx) to nitrogen and oxygen (O2). They are termed “three-way” converters because they trigger three chemical reactions.


A catalytic converter is located directly behind the exhaust manifold or under the vehicle floor pan ahead of the muffler. Some vehicles employ two converters: a “pre-cat” fed directly from the exhaust manifold and a primary converter nearer the muffler.

Catalytic converter of a modern car bottom view.


A catalytic converter consists of an outer stainless steel “shell” containing the “substrate,” a honeycomb ceramic core with hundreds of passageways. 

The substrate surfaces contain platinum, palladium, rhodium, and cerium, and also absorb and store oxygen. As exhaust passes through the substrate, exposure to these catalytic metals triggers the chemical reactions that convert HC, CO, and NOx.

For these reactions to occur at maximum efficiency, a converter must reach at least 750º F. and the air/fuel ratio must be very close to 14.7 to 1, the “stoichiometric ratio” at which combustion completely consumes both oxygen and fuel. 

To maintain this ratio as the engine or powertrain control module (ECM/PCM) rapidly and continuously switches from a lean air/fuel ratio to rich and back based on oxygen sensor data, the converter substrate stores oxygen from the lean ratio and releases it when the ratio is rich.

Drivability Symptoms

If the substrate breaks or collapses, it may block exhaust flow. The vehicle will lose power, especially under load, and may stall or overheat. 

If the substrate surfaces become “poisoned” - coated with a contaminant (oil burned by the engine, silicone in antifreeze escaping into the combustion chamber, or sulfur in poor quality gasoline) - there will be no drivability symptoms. However, any significant decrease in catalyst efficiency will illuminate the instrument panel malfunction indicator light (MIL) in 1996 and newer vehicles.

Catalytic Converter on Ford

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Inspection, Test, and Diagnosis

Vehicles equipped with second generation on-board diagnostics (OBD2) – model year 1996 and later – verify catalytic converter efficiency through a “monitor,” an automatic self-test program run by the ECM/PCM that sets a diagnostic trouble code and illuminates the MIL if converter efficiency falls below an acceptable level. If converter failure is suspected, stored trouble codes should be retrieved with a scan tool.

P0420 through P0439 are converter trouble codes, but the most common are P0420 and P0430, low catalyst efficiency codes for banks one and two. These are Type B, or two-trip, codes that set, capture freeze frame data, and illuminate the MIL when a malfunction occurs during a second consecutive drive cycle.

During normal “closed loop” operation, the ECM/PCM adjusts air/fuel ratio based on “upstream” (before the converter) oxygen (O2) sensor voltage output, alternating from slightly lean to slightly rich and back again. Because the converter stores O2 when exhaust is lean and releases it when rich, “downstream” (after the converter) voltage should change little, regardless of upstream sensor activity. 

To verify converter efficiency, the ECM/PCM compares the voltage responses of both O2 sensors when switching from lean to rich and back. If downstream O2 sensor activity begins to mirror that of the upstream sensor, the monitor concludes the converter’s oxygen storage capacity has diminished and sets the efficiency code.

A false efficiency code can result if the upstream sensor is “lazy,” reacting slowly to air/fuel ratio changes, thereby prolonging duration of both lean and rich ratios. Prolonging the lean cycle provides more oxygen than the substrate can store, so it doesn’t have enough to release during the prolonged rich cycle. 

Consequently, oxygen levels exiting the converter will fluctuate, paralleling upstream sensor activity and triggering the efficiency code. An O2 sensor insufficient switching code may not set because criteria for the codes differ.

It is futile to replace a converter without determining the underlying cause of its failure. Bluish discoloration indicates the converter overheated – temperatures above 1600º F. Overheating can melt or collapse the substrate, plugging the converter, or it can glaze the substrate surfaces over the catalytic metals, destroying efficiency. 

Professional Mechanic Standing Under the Vehicle Lifted on Car Lift Checking the Condition of Catalytic Converter.

Overheating results from incomplete combustion in the engine: unburned fuel in the exhaust completes combustion inside the converter. A poisoned converter generates no heat, so there is little difference between inlet and outlet temperature. Road hazard damage can break the substrate, also increasing back pressure.

Test backpressure by connecting a vacuum gauge to the intake manifold, usually by disconnecting the power brake booster vacuum hose, and measure vacuum at warm idle. Raise engine speed to 2,500 rpm. Vacuum should promptly recover to near idle level. Release the throttle. Vacuum should jump, and then return to idle level. If vacuum increases only gradually or doesn’t rapidly recover, there may be an exhaust obstruction.

To verify that it is in the converter, remove and sight through the converter. If light does not appear through the opposite end, it is plugged. The muffler should then be checked for substrate debris. 

To test converter temperature, remove and plug the fuel pressure regulator vacuum hose to enrich the air/fuel ratio, then measure temperatures with an infra-red thermometer. Outlet temperature should be about 150º F. higher than inlet temperature with the engine warm. To check for a broken substrate, hit the shell with your palm and listen for rattling.


If the converter appears to be functioning, but a catalyst efficiency code has been set and returns when cleared, check for a technical service bulletin issued by the vehicle manufacturer updating ECM/PCM programming to address the issue. False efficiency codes may result from PCM programming making the monitor overly sensitive to downstream sensor activity.


Both federal and state laws regulate catalytic converter replacement. Federal law prohibits replacing a converter unless the original equipment converter failed. Many states require specific documentation when a converter is replaced. Additionally, the replacement converter must comply with emissions standards applicable to the specific vehicle.

Catalytic Converter on Nissan

Vehicles are manufactured to conform to either federal or “California emissions” standards. A vehicle need not be located in California to be a California emissions vehicle. It is the state in which the vehicle was sold new that determines the applicable standard. Thirteen states have adopted California standards. 

The standard to which the vehicle was manufactured is specified on the “emissions sticker” in the engine compartment or on the underside of the hood. In California itself, unique standards apply to replacement converters and only those approved by the state’s Air Resources Board may be installed.

Replacement converters are either “universal,” “direct fit,” or “original equipment” (OE). Universal converters are the least expensive, but are not designed to fit the specific vehicle, so usually require modification of the exhaust system. 

“Direct fit” converters are bolt-in replacements, but contain less catalytic precious metal than OE converters, so have a shorter service life. Aftermarket replacement converters are warranted for 5 years and 50,000 miles for the shell and 25,000 miles for the substrate.

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