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Catalytic Converter (CAT)

The catalytic converter is a device installed in the exhaust system to significantly reduce the emission levels of hydrocarbons (HC), carbon monoxide (CO), and in the case of most newer catalytic converters, oxides of nitrogen (NOx). CO and HC reduction CATs are called Two Way CATs, and CATs which reduce all three emissions are called Three-Way CATs. Your vehicle's Underhood Emissions label can provide you with information regarding the type and requirement of this component.

If your vehicle requires a catalytic converter it must be present, installed and functioning properly. 95% of vehicles on the road in California are required to be equipped with a CAT.

Operation: The CAT is a catalyst, it typically plays no part on how well your engine runs. The catalytic converter only begins working after exhaust has exited the engine's combustion chambers. When hot exhaust gases are forced through the catalytic converter, they contact the catalyst substrate. This causes a rapid increase in temperature, thus burning the exhaust gases even hotter and more thoroughly, ultimately reducing emissions.

In the Three Way CAT, once the HC and CO reduction has occurred by extreme heating, a NOx reduction substrate separates the harmful pollutant known as nitrogen oxides into nitrogen and oxygen.

Catalysts are designed to function for a limited time, and require replacement. How long a vehicles CAT will last depends on how completely the engine burns fuel before presenting it the to the CAT and also on the quality of the CAT. Factory CATs should normally last 7 to 10 years. Aftermarket CATs should work for at least half that. We see CAT failures most often in vehicles which have been poorly maintained and which have produced high emissions in more then one category.

Location: The catalytic converter exterior is usually made of stainless steel. The CAT looks very similar to a muffler except it contains a catalyst substrate within and is normally located in the exhaust system before the muffler and after the exhaust manifold. Most often you will located the CAT underneath your vehicle towards the center of the exhaust system. On certain 4 and 6 cylinder model engines you will see the CAT directly after the exhaust manifold. On these model vehicles the CAT and exhaust manifold may be one unit.

For the Smog Test: During the smog inspection the smog technician will visually inspect the CAT for presence and proper connection. Should your vehicle fail the smog test the CAT may be suspect. A vehicle's CAT usually becomes plugged after a material breakdown within it normally due high exhaust temperatures and rich fuel. After first inspecting the fuel and ignition systems, an intrusive CAT test and Back Pressure/Vacuum test may be performed. These tests will determine whether the CAT is plugged. The important thing here is this... if in fact the CAT is damaged, it is important to ensure a pre-existing problem is not the culprit.

Which Type Should I Choose?


You don't want to damage the new CAT. A CAT melt down usually occurs due to high CO (Rich Fuel), old age, and extreme heat. It is recommend to suspect the CAT after a complete emissions systems diagnosis has been performed.

Symptoms of defective catalytic converters may be any of the following:

  • Symptom: Loss of engine power over 15-20 miles per hour.

    Fault: CAT is plugged up and restricting exhaust flow.

  • Symptom: Strong sulfur (egg) smell from good running engine.

    Fault: Catalyst not completing the burning process properly.

  • Symptom: Rattle being heard from CAT during idle and acceleration.

    Fault: Catalytic substrate broken down and possibly plugged.

How to find out if a catalytic converter is defective?

The most accurate way to find out if your vehicle's CAT is working efficiently is by using an exhaust gas analyzer. Unfortunately this tool is fairly expensive and not designed for home use. You must visit a local smog station and have the smog technician inspect the CAT via the shop's gas analyzer.

California Catalytic Converter Laws and Regulations

The smog stations says my catalytic converter is not legal in California

Since the implementation of the new STAR smog check program, smog stations are very careful in ensuring vehicles are equipped with proper/required catalytic converters. The smog check technician will check your vehicle's catalytic converter for an EO number/stamp. If the EO (executive order) number doesn't not match the correct EO number as indicated on the California Air Resources Board website for Allowed Aftermarket Catalytic Converters, your vehicle will fail the visual portion of the smog check as an "emissions tamper".

My vehicle failed the smog check for CAT Wrong Application

If your vehicle has passed smog checks in the past with a catalytic converter which is not passing now, and you are certain the catalytic converter used to be allowed for use on your vehicle, you might have recourse.

Under current regulations a pre-OBD II (pre-2009) converter can pass the smog check as long as it was installed before 1/1/09, and you can provide the smog station proof of the installation date and proof that the CAT was designed specifically for use on your vehicle (including make, model, year, and engine size). The following is a PDF by the California ARB explaining the new California CAT law. Contact the California Smog Check Referee Office at (800) 622-7733 for more information or to make a smog inspection appointment.

If you can not prove your vehicle's catalytic converter is of proper fit and design, or that it was installed before 2009, the following ARB Aftermarket Catalytic Converter Lookup Table will show you which ARB approved CATs are currently allowed for your make, model, year, and engine size of vehicle.