7 Symptoms & Signs of a Bad or Clogged Catalytic Converter

A catalytic converter is a critical component in every vehicle. It is designed to reduce hazardous emissions discharged into the atmosphere.

We are all well aware of the harmful impacts of combusted vehicle fuels on human health and their impact on air pollution. Commonly hazardous substances include:

  • smell like rotten dioxide
  • carbon monoxide
  • nitric oxide
  • and hydrocarbons.

Instead of releasing these toxic byproducts, a catalytic converter uses chemical reactions to convert them into harmless elements. They’re turned into carbon monoxide and water vapor elements before releasing them into the environment.

Here is everything you need to know about a bad or clogged catalytic converter and the signs and symptoms to look out for!

catalytic converter

What you Need to Know

Admittedly, an engine’s combustion process is not perfect. It may produce bad reactions during the cycle. The heat in the engine and the absence of a converter create specific unwanted gases and annoying rattling noise.

One of the few examples is carbon monoxide (CO). When this poisonous gas is emitted into the atmosphere, it contributes to tropospheric ozone development. Then interacts with methane destruction in the stratosphere. It is harmful to the ozone layer. 78 percent of the air is boosted up and combined with oxygen in particular combinations. Combustion also produces nitrogen dioxide (NO2). High amounts of nitrogen dioxide are likewise detrimental to the human body and the environment. Unburned fuels or incomplete combustion create hydrocarbons, also expelled through the tailpipe. Inhaling hydrocarbons is hazardous to one’s health, especially respiratory issues.

The good news is that catalytic converters are intended to reduce all three harmful pollutants. It is found underneath the car, just in the center of the exhaust system. It sits between the engine and muffler. It goes immediately behind the exhaust manifold before going to the resonator and muffler. It filters out pollutants before they go out of the stream tailpipe.

Make sure to read our guides on deep cycle batteries and basement dehumidifiers.

Two Types

Eugene Houdry was a French chemical engineer. He was the first to patent one in 1950.


There are two types of catalysts: Reduction and oxidation catalysts. Let’s recap what it accomplishes in chemical reactions. It is a compound that causes a chemical reaction to occur quickly without being consumed. In other words, it is something that facilitates the reaction’s development.

Reduction and oxidation catalysts are made from a ceramic structure covered with a metal catalyst such as platinum or rhodium. It’s the reason they’re a costly component of a vehicle. These metals are arranged in a honeycomb pattern. This arrangement exposes as much surface as possible while still using the least quantity.

The reduction catalyst is made up of platinum and rhodium. It’s the first stage in the process. When a nitrogen oxide or nitrogen dioxide molecule comes into contact with the catalyst, a nitrogen bond is broken. It forms a harmless diatomic gas (O2) molecule alongside the nitrogen molecule.

Oxidation is the next stage. It burns unburned hydrocarbons and carbon monoxide over a palladium or platinum catalyst, oxidizing them. It utilizes the oxygen molecules in the exhaust, and the oxygen is freed up during the first stage (reduction stage). So there are 2 ways there: The first is to bond carbon monoxide to produce carbon dioxide. The second is to oxidize unburned hydrocarbons as they pass through the catalyst.

Further, it uses oxygen sensors to monitor the level of oxygen in the exhaust system. These sensors are interconnected to an automated control system. It changes the air-to-fuel ratio by adjusting the carburetor. It lets the car’s fuel mix with more or less air before it enters the cylinders. As a result, it ensures adequate oxygen in the exhaust to oxidize unburned hydrocarbons and carbon monoxide.

exhaust system of a car

What do they do?

Their primary job is to convert toxic nitrous oxides and carbon monoxide into less harmful substances. It does the same for unburned. These harmful substances are turned into nitrogen and carbon dioxide instead. You’ll also get water coming out. It is divided into two stages: reduction and oxidation.

When hazardous components pass through the catalyst, they split up and develop into less toxic gases before exiting the pipe streamlines. When the gas enters the system from the input (pipe linked to the engine), it is pushed over the catalyst. It generates chemical reactions and breaks down pollutants. The less hazardous gases then go all the way to the output (which is linked to the car’s tailpipes) and finally out into the air.

Recent versions are engineered to be highly efficient. Indeed, premium models have manifested increased horsepower. Catalytic converters in most modern vehicles have reduced American emissions pollutants by an average of more than 70%. The bottom line is that it is undoubtedly one of the most significant advances in automotive history.

small car outside

7 Signs It’s Clogged

These components are critical in the conversion of harmful pollutants into safe gases. It is the safety defense against air pollution from automobiles positioned in a vehicle’s exhaust system.

Even though they are designed to endure a long time, they can get contaminated or clogged. They can also get damaged or overheat due to extended exposure to various contaminants. These issues will result in slow engine performance or engine failure.

Here are some common signs of a clogged catalytic converter.

1. Increased Emissions

Its function is to burn unburned fuel or gases that exit the engine, resulting in low carbon emissions. When your vehicle emits increasing carbon emissions, it indicates a clogged converter. It will then produce black smoke coming out of the exhaust resulting in a failed emission test.

2. Poor Performance

When carbon buildup inside becomes excessive, it causes a partial blockage within the catalytic converter. Alternatively, the internal system begins to melt due to the excess heat of unburned fuels. It can result in immense back pressures that reduce performance. Your car will shake regularly, and there will be backfires or unexpected bursts of pressure and engine stalling. It won’t go faster than 30-40mph.

3. Misfiring

Another thing to pay attention to is misfiring. If it happens, fix the problem as soon as possible. A misfire can damage the component due to unburned gas exiting the combustion chamber. Instead, it will discharge it into the exhaust system.

picture underneath a car

The following factors may also cause misfiring:

  • Bad coils
  • Spark plugs
  • damaged/dirty fuel injectors
  • Busted head gasket
  • Other possible mechanical issues
  • Check engine light is on.

Oxygen sensors in modern cars analyze exhaust gas levels to monitor the effectiveness of the component. If the sensor detects that the gas has not been well converted, the check engine light will illuminate.

check engine light is on

4. Bad Smell Coming Out from the Exhaust

Fuel contains many sulfur particles that convert to hydrogen sulfide during combustion. The stinky sulfide is meant to be converted into odorless sulfur dioxide by a catalytic converter. It does not occur if the system is faulty or clogged. You will begin to smell like rotten eggs as unburned gasses escape your exhaust, including smelly hydrogen sulfide.

5. The Exhaust Produces a Rattling Noise

A rattling sound coming from your engine is because of components in the honeycomb-shaped chamber inside the catalyst breaking. It can be due to overheating or damage triggered by too much use of a rich fuel mixture. You’ll probably notice it when starting or switching off your car since the damaged pieces are rattling around inside.

6. Reduced Fuel Efficiency

The piston will be stressed if it is clogged. It is due to a system blockage that allows back pressure in the exhaust gas, resulting in reduced acceleration. You’ll then be forced to press harder on the gas pedal. It means you’ll have to fill up your gas tank more frequently, which seems uneconomical.

7. Hard Start and Overheating

When completely clogged, the engine cannot breathe because the exhaust gases have nowhere to go. It tends to travel back up into the engine, causing it to overheat and the head gasket to blow. Excess heat will overheat the cooling system, reducing its ability to cool the engine.


What happens when a catalytic converter fails?

Catalytic converters do go bad on cars, so what happens then?
The converter blocked the exhaust system and increased the pressure way too much.

Unburned fuel and other particles block the honeycombed mesh, causing pressure to build up.

The temperature gauge will register somewhat higher because the exhaust gas remains in the engine for a very long period. It causes it to overheat.

The automobile will not go at a higher speed for much further, potentially not exceeding 30-40 mph.

Overheating will become a significant problem.

No start or hard starts are common. Stalling is also among the significant issues to look out for.

How do you know if your catalytic converter is going bad?

A converter may sometimes survive ten years or more. Over time, it deteriorates and no longer converts the exhaust gases as effectively as it should. You will begin to notice several issues with the performance of your car. Here is the list
Car fails in the emission tests.

The car pushes black smoke out.

The vehicle is juddering while accelerating.

Struggle going over 30-40 mph.

It emits a sulfuric smell of rotten eggs from the exhaust.

The car tends to overheat.

You’ll spend too much on fuel compared to before.

The check engine gauge indicator turns on.

It produces annoying rattling noise in the exhaust.


Reduced performance/low power

Going up a hill is difficult due to a lack of power.

Failing components. It could be the radiator or water pump gasket. You may also have issues with cooling system hoses.

What does a bad catalytic converter sound like?

If it has come apart internally, it will produce a rattling sound. There is no distinct beat to the rattle noise. It may sound like rocks/pebbles bouncing around in the exhaust. It is because the honeycomb has disintegrated inside the pipe. When you open it, you will be startled by melted chunk-like fragments.

Can a Bad catalytic converter cause a p0300 Code?

Maybe your car’s check engine light illuminates with the dreaded p0300 code. It means your engine is experiencing intermittent misfires. According to the p0300 code, at least two or more cylinders encounter misfiring.

Random misfires happen for a variety of causes. The p0300 arbitrary misfire code is due to a faulty catalytic converter. It prevents the exhaust from breathing correctly, causing unburned gases to ignite within.

When p0300 develops, it must be repaired as soon as possible since it may cause further damage to your engine. P0300 occurs when worn-out spark plugs, torn spark plug wires, or a defective ignition coil.
P0300 Indications:
The check engine light is on and flashing.

Lack of engine power

A nasty fuel smell is coming out from the exhaust.

The engine runs rough and shakes.

When you accelerate, you will experience jerking.
flashing check engine light

Can a clogged catalytic converter cause a misfire?

Fuel combustion supplies the energy to power the engine. Effective fuel combustion is critical to engine functioning. A misfire happens when insufficient gasoline is burnt in a cylinder. And it could happen for many reasons. It could be a faulty ignition or a clogged exhaust system.
Typical Issues that Cause Misfires:
Cracked plug

Bad ignition coil

Old fuel injector

Weak fuel pump and fuel filter

Dirty throttle body

Bad airflow sensor

Vacuum leak

Misfiring should not be taken for granted, especially if you often travel long distances. Ignoring the mistake might result in engine failure or catalytic converter damage. You don’t want risky driving conditions.

Unclogging it in 6 Easy Steps

A catalytic converter (cat-con) regulates the exhaust emissions of an internal combustion engine by catalyzing a chemical reaction. It contributes to reducing the environmental effect of cars.

A clogged cat-con loses its capacity to filter out harmful gases. However, it is pretty expensive. It is up to you if you want a new replacement or to clean the existing one.

How to unclog a catalytic converter

  1. You may get a cat-con cleaner in automotive supplies or browse online to find the best brand for your vehicle.

    Depending on the model of your car, you can follow the complete instructions provided. If it doesn’t work, there is another way to unclog it. You will need to know cars.

  2. Remove the component.

    You will need a degreaser, a high-pressure power washer, and cleaning cloths. You may also require a car jack and supports to lift the vehicle. Don’t forget to use safety equipment. Don’t forget to cool down the vehicle before you begin.

  3. Locate the catalytic converter

    The cat-con is located on the car’s underside, in the center of the exhaust system. Using a wrench, unscrew the bolts. Stubborn bolts may require oil to grease and soften. If it sounds like there are damaged components, shake the converter.

  4. Blend the degreaser and hot water in a container.

    Soak it for approximately 1-2 hours. When finished, remove it from the mixture and rinse it with clean water. Allow it to dry before reassembling. Remember to reinstall it in the same manner that you removed it.

  5. Perform a quick test drive to ensure that everything is in place and working.

    Re-test the converter after test driving. If the result falls within the appropriate ranges, it indicates that the blockage has been cleared. If the unit fails too badly during testing, the only alternative is to replace it.

Keep in mind that they are a critical component for the proper performance of your car. Call a mechanic to do the job for you to guarantee proper unclogging. After all, hitting the road safely should be your top priority.

So rev up your engine with a safe and clean exhaust!

small car about to be lifted

Leave a Comment

Your email address will not be published.