10 Signs of a bad or clogged catalytic converter

Catalytic converters are important parts of a car, and you don’t want it not working. In fact, it’s one of the more expensive parts that can need replacing.


The most common symptoms of a bad or clogged catalytic converter are the P0420 engine code appearing, bad smells, and a check engine light. Knowing that a bad catalytic converter can also show signs by activating a p0300 code is important.

Here’s what we’ll be covering in this article:

10 Signs of a bad or clogged catalytic converter

catalytic converter

When your car stops working effectively, it is the first sign that something is wrong with it. Detecting the reason is the more complicated task you will face afterward. If you are facing problems with acceleration, misfires, or poor fuel economy, that means your catalytic converter needs to be examined.

Other than low engine efficiency, you may spot:

  • strange noises
  • bad smells
  • The P0420 code
  • or even smoke coming from beneath your car.

This type of weird activity your car might be doing is a clear sign of a clogged catalytic converter.

1. Your car’s engine does not function well.

If you have recently observed that starting your car has become more difficult than it was, the clogged catalytic converter might be the cause. It is an integral part of a car’s emission system.

The engine fumes cannot exit the system properly when it gets clogged. As a result, the fuel gets less air than is needed for the engine to function properly. Your engine is at great risk of failing in the future with this improper balance of gases and additional pressure inside the emission system.

2. You see the P0420 code.

The P0420 code indicates the low efficiency of the component. In most cases, the reason why you see this code is a bad part. Sometimes it occurs because of misfires or defective oxygen sensors. In both scenarios, it is better to hurry up with the repair. Driving with this code will lead to further internal damage to your car.

3. The car’s fuel runs out faster.

Every driver’s nightmare is spending more money on fuel than they have in the past. With that regard, it is more likely that the moment you start filling your tank more often than usual will not pass unnoticed. The case is that a damaged catalytic converter causes the blockage of air supply to the engine.

When the oxygen does not dilute the fuel correctly, the engine faces difficulty with acceleration and consumes more fuel than usual for that task. In rare cases, a blockade leads to the opposite reaction of little fuel consumption. Maybe you have noticed that symptom. Do not try to make a profit from it because the damage to the engineis also inevitable, in that case.

4. Your car fails the emission test.

In some countries, drivers are obliged to do emission tests once in a while to check the serviceability of their vehicle’s emission system. You can also perform the test on your own whenever you feel the need.

This test is one of the best ways to clear up all doubts concerning your catalytic converter. Everything is simple. If the car fails the test, then the part is damaged.

5. Difficulty with acceleration

A clogged cat converter also prevents fuel from getting to an engine. For this reason, the car may jolt or shut down altogether when you stamp on a pedal. It becomes challenging to accelerate, which can signal a defect.

Nonetheless, you should also consider more than one symptom to diagnose a bad cat converter because acceleration difficulty can also be related to engine malfunction.

6. A bad smell coming from the engine.

One of the distinguishing signs of a failing catalytic converter is the smell of sulfur, which is somewhat similar to the rotten eggs odor. That happens when badly filtered exhaust fumes are pumped out of the engine.

The problem here is that your cat is clogged, and the fumes you may inhale while driving endanger your health and safety. Exhaust fumes are stuffed with carbon monoxide, making you lose consciousness while driving. That is why smelling a bad odor from the engine will not only point out a clog but also motivate you not to delay your vehicle’s repair.

7. Your car fails the temperature test.

This test you can easily perform on your own. The only thing you need is an infrared thermometer to measure the temperature. The trick is to drive the car for a short period of time so that it will reach its operating temperature.

The next step is to park it on a flat surface and turn on parking or neutral mode, depending on your car type. Finally, measure the temperature of the front and back pipes of the catalytic converter. If the results do not differ significantly, that is a symptom of damage in the filtration area.

8. You hear noise from underneath your car.

Any suspicious noise you hear from beneath your car is a sign of probable damage. Noise is usually caused by a piece of honeycomb that has become loose inside the catalytic converter.

If you put off the repair, it may become stuck in the exhaust pipes while driving and cause your car to stop in the middle of the road. You will need to spend extra money on car transportation in this critical situation.

9. The component is discolored.

A clogged catalytic converter creates excessive pressure inside the emission system. This pressure also increases the inner temperature, turning the housing blue or any other color.

The alternative reason for discoloration might also be the high temperature within the catalyst. The engine fails to burn fuel sufficiently when it’s clogged. That is when the housing surface turns an unnatural color and points out the issues within.

10. The lights on the car’s dashboard are on.

Most modern vehicles are designed to alert the owner of potential emission system problems. That is why cars are equipped with dashboards that monitor the level of oxygen mixed with the fuel. When there is an imbalance, the lights on the dashboard automatically switch on to draw your attention to the possible issue.

What do they do?

Even the luckiest car owners are not immune to clogged catalytic converters. No stress is needed if you are new to the world of automobile service and know nothing about this part of your vehicle. A catalytic converter is a special device that:

  • converts toxic components of a car’s exhaust gases into safer ones.
  • burns them up
  • reduces the amount of pollution
  • makes your car able to pass inspections 

You’ll find it between the muffler and the engine.

Usually, a car engine produces nitrogen oxide (NO) and carbon monoxide (CO) that sufficiently pollute the air. For that reason, most automobile companies equip gasoline and diesel engines with these types of filters so that the cars become more environmentally friendly.

catalytic converter and DPF

The neutralization process of exhaust gases is not a complicated one. A catalytic converter is just a box between the engine and your car’s tailpipe. The catalyst inside adds oxygen to toxic carbon monoxide and removes it from nitrogen oxide.

As a result, exhaust fumes are much safer for the environment. The material of the catalytic converter is also important in the filtration process.

The equipment of that kind is usually made from platinum and rhodium—expensive metals that often get stolen. The other unfortunate thing common for this component is that they sometimes get clogged.

Every catalytic converter will eventually go bad. Over time, the carbon deposits congest the inside of the cat and force it to work inadequately. Another reason for faulty cat functioning is unburned fuel.

When excessive fuel reaches the catalytic converter, it ignites and elevates the temperature to the extremes. High temperatures will eventually cause an inner meltdown. It also means worse fuel efficiency.

small car outside

Sometimes your vehicle can have issues affecting its functioning. The engine will get a harmful fuel mixture if the oxygen sensor works poorly. As a consequence, it will increase the temperature inside the emission system.

If you are a fan of short distances, you should consider taking your car for a long ride once in a while. Do not use your vehicle for a brief period. In that way, it does not heat up enough for good functioning.

If you already suspect that the catalytic converter is clogged, consider the common signs of that issue. Otherwise, they may not last the lifetime of the car.

What happens when it goes bad?

With catalytic converters, it is often the case that they go bad. The older your car gets, the more chances you have of facing that problem. It is similar to a certain part of a pipe in your household.

If something gets stuck there, the water supply is disrupted. The same thing happens when the component is clogged. It fails to discharge noxious gases from the engine and shatters its proper functioning. 

A defect in one area of your vehicle is like a cancerous tumor that will spread later on without proper diagnosis and treatment. This worst-case scenario may happen if you don’t notice signs of a clogged catalytic converter in time. To ensure the safety of your car, the list of ten symptoms will come in handy. This way, you can look out for them.

Performance issues could also be caused by other parts of the emissions system, so make sure to check those.

What does a bad one sound like?

catalytic converter

A catalytic converter should not produce unnatural noises for vehicles. You will hear a rattling, ticking, or knocking sound when it is damaged. 

The rattling sound is the one that a cat converter makes when the honeycomb is lost because of age or mechanical damage. It is easy to hear a rattling noise when the cat has not reached a high temperature. When it is heated, it loses sonority. Unfortunately, a damaged honeycomb means that the part is beyond repair and needs to be changed.

Often, a rattling sound comes from an unsteady heat shield. That happens when bolts are too old to keep the heat shield fastened or when they get loose on their own. To remedy the situation, you need to tighten it back up. 

The reason why you hear a ticking or knocking sound beneath your vehicle can be a leak or exhaust fume. Those noises get louder when you accelerate your car. If you spot an exhaust leak, it’s best to leave it to the mechanics to weld.

Can they cause a p0300 code?

It certainly can. The p0300 code designates random misfires within the cylinder. This code usually appears when the car faces complications with the ignition or has an inner engine malfunction. One of the causes of the p0300 code is a faulty catalytic converter.

The symptoms of this code are very similar to those listed for a clogged cat converter, but the code generally points out engine problems. That is why it is easy to get confused about why the p0300 code appeared. You should always remember that a bad catalytic converter is often the cause of the whole system contamination.

When your car shows a p0300 code, this is the right time to consider more profound repairs. The damage is more serious if your vehicle has all the symptoms of a clogged catalytic converter and a p0300 code. It is not only unsafe to drive on the road with such a problem; it also increases the likelihood of a collision.

Can a clogged catalytic converter cause a misfire?

exhaust system of a car

Yes, it can. During the combustion process within the engine, the cylinder absorbs the mixture of fuel and oxygen. When there is a blockade, exhaust fumes stay inside the system.

As a result, the amount of oxygen is scarce. The cylinder starts to absorb exhaust fumes. The combustion process requires oxygen, and the engine misfires when it does not get it.

The vehicle’s computer often discovers the problems within the engine system when a misfire happens and throws the diagnostic trouble code. That means it is time to check the part when you see the check engine light turned on.

Here are some reasons for a clog:

  • Oil or antifreeze entering the exhaust system
  • Unburned fuel entering the exhaust
  • Short trips
  • Improper additives and fuel


Unclogging a catalytic converter is certainly easier than the diagnostic part. Once you are sure it is clogged, you should find out how bad it is. If the damage is minor, you can try to repair it yourself. 

catalytic converter on light background

Running your car too slowly is what sometimes causes minor cat clogging. Emission vehicle systems and an engine need a certain temperature for efficient functioning.

The best temperature for a catalytic converter is in the range of 800 °F to 1832 °F. You can try to drive your car faster than you usually do so that the heat will burn off all undesirable sediments in the system.

ProblemCost to Fix
Engine rebuild or replacement$4,000 – $8,000
Faulty catalytic converter$1,000 – $2,500
Fuel injection system repair$800 – $1,500
Hybrid battery replacement$4,000 – $8,000
Suspension failure (shock absorbers, etc.)$1,500 – $3,000
Timing belt replacement$1,000 – $2,000
Transmission failure$3,000 – $4,000
Turbo or supercharger failure$2,000 – $5,000

The second method includes using a cleaner. Ensure your car has 15 liters of fuel before pouring the cleaner into the fuel tank. The next thing you should do is to drive a car for about 30 minutes at no less than 2500 rpm till the fuel runs out. Pay attention to the temperature of an engine so it won’t rise too high.

This method will be successful when you no longer have any problems with acceleration. Fewer fumes from an exhaust pipe are also a good sign. It is an efficient way to clean minor clogs.

Dura Lube Fuel Additive

Here’s the product we recommend for the purpose.

NameSevere Catalytic and Exhaust Treatment Cleaner Fuel Additive
BenefitsIncreases fuel economy
BrandDura Lube
Country of Origin‎USA
Item Form‎Liquid
Product Dimensions‎2.62 x 2.62 x 9.12 inches
Weight16 fl oz


  • It’s a great option for minor clogs.
  • It’s a very affordable treatment.
  • It might save you from having to replace the device.


  • It may not be able to fix major clogs.
Dura Lube Additive
It's a good fuel additive.

For a more considerable blockade, try the following procedure.

picture underneath a car

The other method is the more complicated one, which involves removing the catalytic converter. To avoid injury, make sure the exhaust system is cool. Then put your car on jack stands and get rid of the oxygen sensors.

After removing the catalytic converter, check it for a rattling noise. If you hear any sounds inside, stop the unclogging procedure and replace it with a new one. The following steps are to wash it with a pressure washer and plunge it in hot water with a degreaser. Finally, let it dry and reinstall it afterward.

Those methods are useful when the damage is not significant. If yours has serious mechanical damage, manual cleaning won’t work. In that case, the only solution is to buy a new one. Considering that they are made of expensive metals, it would impoverish your purse for no less than $1000. Make sure to look out for clogs to and get the car’s best mileage.

Be sure to be on the lookout for a decrease in power, so replacement of these palladium-filled parts won’t be necessary. You’ll even be able to pass the emissions test without problems upon inspection.

small car about to be lifted

We will be happy to hear your thoughts

Leave a reply

Passion Plans
Login/Register access is temporary disabled