Maintenance Check: 6 Signs That Your Catalytic Converter Needs Replacement

Worldwide, ambient air pollution is considered to be horrible proof of growing urbanization. The World Health Organization (WHO) estimated that about three million people die every year as a result of the increasing gas emissions in the atmosphere from factories, power generation, and transportation machinery like cars and motorbikes.

The pollution problem peaked in the 1960s and 70s, during a time when industrial factories and skyscrapers sprung up almost overnight in cities and provinces. The growing number of cars and motorbikes in the streets is also considered to be one of the contributing factors as to why air pollution has continued to run rampant.

These were the circumstances that eventually led to the passing of the Clean Air Act in 1970 which was designed to control air pollution on a nationwide scale. Due to strict regulations being imposed by the U.S. Environmental Protection Agency (EPA), automobile manufacturers were forced to find a way to deal with the harmful exhaust emissions and this is where catalytic converters were introduced to the public.

Look What This Cat Can Do

For compliance purposes, all cars that were manufactured from 1975 onwards for US sale had to have a catalytic converter installed in the exhaust system. But what does this piece of metal do, exactly?

A catalytic converter, or “cat” for short, was created to reduce the amount of harmful gases such as carbon monoxide, nitrogen oxide, and hydrocarbon being emitted from a car’s exhaust. It contains noble metals on an inside honeycomb which upon coming into contact with the noxious gases, converts them into carbon dioxide and water. Some of the metals acting as catalysts in this event are platinum, palladium, and rhodium.  Surprisingly, these precious metals make catalytic converters a target for petty thieves.

Catalytic converters can be found near the engine under the driver’s seat. It is illegal under Federal law to remove the cat from your car unless it has failed a state or local emissions test and legitimately needs to be replaced.

Signs That Your Cat Is Nearing Retirement

Catalytic converters experience issues as they age just like any other car part. The following symptoms listed will help you to determine if your cat is nearing retirement age and subsequent replacement. has a huge selection of replacement catalytic converters.  [shop now]

  1. The “Check Engine Light”

catalytic converterCondition 1 – Blinking CEL (engine misfire):  If your check engine light is blinking, it indicates a serious engine misfire, usually accompanied by loss of engine power and hesitation. When this type of problem occurs, unburned fuel passes into the exhaust system where a catalytic converter’s noble and precious metals can quickly overheat and become damaged if the problem isn’t addressed.  Do not drive your car any distance with a blinking check engine light and have the problem addressed immediately or it could ruin your catalytic converter.

Condition 2 – Solid CEL:  On the other hand, if your OBD II shows a P0420 code, this is a sure indication that either your cat converter is not operating at maximum efficiency and may need to be replaced or the O2 sensors have a problem. The P0420 diagnostic code stands for “Catalyst Efficiency Below Threshold (Bank 1)”. If you see this code, all the more reason to have your car brought in for inspection as soon as possible.  If you have a V6, V8, V10, or V12, you can also get a P0430, which would be the same code, but for the second catalytic converter on true dual exhaust systems (such as many BMW’s).

  1. The Car Isn’t Accelerating Properly

A catalytic converter can become so clogged with carbon and smog to the point that exhaust gases have difficulty passing through the muffler and exiting out the rear of the car. Since less air is able to exit, less air will be able to enter which can affect the performance of the engine and cause it to stall while you’re driving.

An easy way to identify if your cat is clogged is by having someone rev up the engine to around 1800-2000 RPM and hold it steady. Put your hand near the tailpipe and check for a good push of exhaust air. If the airflow seems sickly and slow, your cat is most likely clogged and you’ll need to have a mechanic take a look. Some cars, such as newer Saabs with the Tech 2, have a Catalytic converter checking phase in the dealer scan tool that can help diagnose a clogged catalytic converter.

  1. The Exhaust Fumes Smell Bad

If the pungent odor isn’t coming from the rotten eggs you’ve accidentally broken in the back seat, then maybe your catalytic converter is to blame. A plugged cat may cause emissions to smell like sulfur or rotten eggs whenever you start your car or as you drive along the highway.

A small amount of sulfur is naturally present in the fuel. This sulfur is normally converted to sulfur dioxide (which is odorless) as it passes through the catalytic converter. However, if the emissions exiting the exhaust still smell like sulfur (rotten eggs), it could be an indication that the cat is clogged or the catalyst honeycomb inside has corroded.

  1. You Hear Rattling Noises While Driving

If you hear a sound akin to that of a baby’s rattle or stones banging inside a metal can when you start the engine, your catalytic converter could be to blame.

The rattling noises can sometimes be caused by the small pieces of honeycomb catalyst that fell apart due to a collision or sudden impact. Cat converters are solidly built, so the metals will not fall apart by themselves – only an external force can cause them to do so.

The honeycomb is the place where the filtering, converting, and expelling of gases happen inside a catalytic converter, so if any piece of it has become damaged it will not be able to efficiently control and convert engine emissions.

  1. Increased Tailpipe EmissionsBad Catalytic Converter

The noble metals inside a catalytic converter can also become contaminated by substances such as motor oil or any type of coolant. If you see an increasingly larger amount of carbon emissions spewing from the tailpipe, it could mean that the cat converter is not doing its job and needs to be replaced.

Have it checked by a mechanic right away to avoid damaging the entire exhaust system.

  1. You Failed an Emissions Test

32 out of 50 states in the US require car owners to have a smog check certificate for their vehicle. To get one, you must pass an automobile emission test, which can be done at local inspection stations in your area.  Failing an emissions test may mean that any of your muffler, pipe or entire exhaust system needs replacement. Most commonly, however, the primary culprit tends to be a bad catalytic converter. Replacement cats can cost over $1000, so it’s imperative to do regular checkups in order to keep your vehicle in prime condition.

If you fail an emissions test, you cannot register the vehicle which means you can’t get license plates and subsequently won’t be able to drive the car (anywhere except to an auto center or repair shop).

Important Note When Shopping For A Catalytic Converter can find a very reasonable aftermarket catalytic converter that will allow you to pass your state emissions with your European car.  However, if you specifically live in California, you are subjected to much stricter rules.  Only OEM genuine dealer CA-compliant cats are allowed to be fitted, and all aftermarket catalytic converters are banned regardless of their functionality, even if they are OEM.

In New York, there are still strict laws governing catalytic converter installation, but not as strict as California.  In many cases, an approved aftermarket catalytic converter manufactured by quality brands such as Bosal and Magnaflow will be listed as 49 state legal, meaning you can install it in any state other than California.  These must be EPA and CARB approved for legal installation.  The only hangup is that if you live in New York and your car has a California emissions tag under the hood, then you will not be able to install one of these 49 state legal cats and pass in New York.  If you are in New York and looking to buy a car from California, keep this in mind.

Writer – Ailey Henry of

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