catalytic converter 1

Catalytic Converter Symptoms, Causes & Replacement Cost

In Engine by Magnus Sellén2 Comments

Do you have problems with your car and suspect that the catalytic converter is defective?

Catalytic converters are known to fail on many cars and they are often very expensive, especially if you are looking for a brand new one.

The diagnosis of a defective catalyst is very important to ensure that you do not replace a fully functioning catalyst because of the replacement cost.

In this article, you will learn about the most common symptoms and causes of a bad catalyst.

Signs of a Bad Catalytic Converter

There can be many symptoms of a bad catalytic converter because if the catalyst becomes clogged, it will severely affect the engine’s performance. If you would like to learn more about how to clean a lightly cleaned catalyst, read our article: How to clean a catalytic converter without removing it.

Here are the most common symptoms when it comes to a damaged or clogged catalytic converter:

1. Check Engine Light

flashing check engine light

The check engine light, one of the most obvious indicators of a malfunction in your car, is also one of the indications of a bad catalytic converter. When checked by a compatible device, it gives a catalyst fault code which must be decoded with the help of the operating manual.

In most cases you will receive a P0420 or P0421 code. These are the codes that appear when the engine control unit has detected a bad catalyst. If you would like to know more about these codes, you can find our other article here: P0420 Code.

In the event that the engine control unit does not display a check engine light, a vacuum gauge can be used to detect a fault. Connect the pressure gauge to the intake manifold line, then start the engine, let the engine idle and measure the pressure gauge reading. The measured value should normally be between 16 and 21 inches of mercury content. Then increase the engine speed to about 3000 rpm and wait, then check the needle of the gauge after a few seconds. It should return to approximately its original value. If it returns slowly or becomes low, it means that your catalytic converter is blocked.

Secondly, the catalyst can also be checked manually with an IR thermometer. Start the car and let the engine run until it has warmed up completely. Once the engine has warmed up, lift the car with a jack or other support and then measure the inlet temperature of the catalytic converter. Then measure the outlet temperature of your catalytic converter.

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Compare both the intake and outlet temperatures. If the outlet temperature is slightly lower than the inlet temperature, it means that your catalytic converter is OK, but if there is a big difference between the two, it means that your catalytic converter is defective.

If you experience a Check engine light on your dashboard, make sure to read the fault codes with an OBD2 scanner to see if you get the P0420 code, and make sure to repair any other fault codes before replacing your catalytic converter.

2. Acceleration Lag

If you observe delays when accelerating or notice that your car does not accelerate properly and loses power when driving in steep areas, while all other parts like spark plugs, filters etc. are fine, then it is your catalytic converter’s fault. Mechanics usually misdiagnose the fault of the catalytic converter with other components that need to be replaced unnecessarily.

You can also check this yourself. Keep your hand on the exhaust pipe and ask someone to push the race paddle to increase your RPM to 2000. If you get a hot feeling in the exhaust, it means that your catalytic converter is choked.

3. Poor Fuel Average

Low fuel economy is one of the symptoms of a poor catalyst. Due to improper combustion of the fuel, it reduces the overall fuel efficiency of the car, resulting in increased costs.

4. Sulfur Smell

When catalytic converters clog, they restrict the airflow of exhaust gases so that the engine of your car runs on a rich air-fuel mixture. The combustion of this excessive mixture causes an odor like rotten eggs. This smell smells like burnt sulfur, which is a very clear indication of a malfunction of your catalytic converter.

5. Discolored Housing

The outer surface of the converter discolors due to overheating, which causes a rich mixture of unburned fuel to enter the converter. This mixture then burns in the converter and melts its inner material, causing blockages. This clogging restricts the flow of gas through the converter and creates a back pressure that affects engine performance. Thermal damage can also occur when hot converters come into contact with the cold area, damaging their parts.

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6. Starting Problems

If the catalytic converter is clogged, the engine gets too much fuel that the engine cannot burn. This excess fuel ignites in the exhaust, which causes the car to have difficulty starting.

7. Increased Emissions

If your catalytic converter is choked, it will hardly work anymore. Emissions from the car will not pass through the catalyst in the converter, so there will be no conversion of gases with a high pollutant content to gases with a low pollutant content, resulting in high emissions.

8. Engine Misfiring

When the catalytic converter is clogged, it restricts the flow of oxygen in engines, and engines require a lot of oxygen for proper fuel combustion. This reduced air intake leads to overheating due to an excessive amount of unburned gases caused by spark plug misfiring.

What is a catalytic converter?

catalytic converter causes

The catalytic converter is a device located in the exhaust system of a car. The catalytic converter consists of a catalyst that helps convert the pollutant gases produced by the car into less harmful pollutants. The structure of the catalytic converter has the shape of a honeycomb.

The purpose of the honeycomb shape is to ensure that the pollutant gases flow through the more exposed surface and are converted into fewer pollutant gases at the maximum. Platinum, despite being one of the most expensive products, is normally used as a catalyst in catalytic converters. However, in some cases, rhodium and palladium are also used.

Since combustion takes place in the engine, all the burnt gases flow at about 800 degrees through the exhaust pipe, which passes through the catalytic converter, thus affecting the exhaust gas flow of the engine. A perfect converter usually lasts up to 10 years. The honeycomb structure of the catalytic converter starts to slowly suffocate, which affects the engine’s performance.

It is sometimes difficult to find out that a car’s catalytic converter is not working perfectly, because the effects of a bad converter are about the same as those of engine problems such as valve or spark plug problems. But if the same worn basic engine is repaired and you still find that your car is either still missing or is shut down, then your catalytic converter car must be checked out.

Causes of a failing catalytic converter

Normally, catalytic converters last for a long time, perhaps even an entire vehicle life, but a worn-out engine can easily destroy your catalytic converter.

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Leaky exhaust valves that allow unburnt fuel to flow into an exhaust system that melts the inside of the catalyst, worn spark plugs, weak ignition, misplaced timing chain, worn pistons, or leaking intake manifolds can be some of the reasons for poor catalytic conversion, as they destroy the inner layer of the catalyst.

It is important to always repair these things before replacing your catalytic converter.

The most common cause of a bad catalyst is misfiring. So make sure that you have fixed any problems with your engine before replacing the catalytic converter.

Catalytic Converter Replacement Cost

The cost of the catalytic converter itself is often between $100-1500 depending on whether you are looking for a universal catalyst or an OEM part. The labor cost for the replacement is often between $50-400.

If you are looking to weld an aftermarket universal catalytic converter to your car, make sure that the welds are 100% sealed, and be prepared for problems and code P0420. My recommendation is to always replace it with an OEM catalyst to reduce the risk of future problems with it. Catalysts are very sensitive parts and must be first class.


So as soon as you experience any of the above symptoms, you are strongly advised to have your catalytic converter replaced before it further destroys the performance and engine parts of your car and puts you in more trouble: for instance, getting the entire car repaired instead of just one expensive catalytic converter.

Catalytic converters cannot be cleaned once they are damaged, because once they overheat, the entire catalyst inside is destroyed, so all that remains is to replace them.

If you have further questions about catalytic converters, feel free to ask your questions below.

2 thoughts on “ Catalytic Converter Symptoms, Causes & Replacement Cost ”

  1. Hi, I got my engine light on last month and I used OBD II to scan the computer which later came out the code is P0430, catayltic converter problem too. However, the light will soon off like running 200 km without touching anything. After, few hundred km running again, it turn it on again and turn it off automatically later as same as above mentioned. This kind of on and off was appeared more than 4 times already. The catayltic converter was replaced 3 years ago with only about 50000 km on it. Could you please to tell any other problem comes out to my case. Thanks.

  2. hey guys, I bought a new 2005 Mercury Grand Marquis LSE, 300,000 miles, check engine light po171,po174 and a couple more misfire lights. We have replaced every part that may affect the light to no avail. Every thing but catalic converters. Could converters
    be causing my problem? The car was brand new, full synthetic oil, full synthetic tranny, full synthetic rear end, and brake fluid.

  3. Due to very high demand and a high amount of comments, you might have trouble getting your comment answered by me. If you want to get fast answers from a certified automotive technician you should ask your questions here: Ask A Mechanic

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