8 Signs of a Bad Catalytic Converter (& Replacement Cost)

The catalytic converter ensures the exhaust gases leaving your exhaust pipe is clean. Here's how to tell if your catalytic converter is bad

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

Catalytic converters are known to fail and 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 expensive replacement cost.

In this article, you will learn about the most common symptoms of a bad catalytic converter, location, function, and replacement cost. Let’s take a quick look at the signs first:

The most common symptom of a bad catalytic converter is a sulfur smell when the engine is warm together with a check engine light on your dashboard. You may also notice engine performance issues like acceleration lag, higher or lower fuel consumption, and a misfiring engine.

While these are far from all possible signs, it’s the most common ones. Here is a more detailed list of the 8 most common symptoms of a bad catalytic converter.

Bad or Clogged Catalytic Converter Symptoms

1. Check Engine Light

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 catalytic converter trouble code which must be decoded with the help of the operating manual.

The most common trouble code when it comes to a bad catalytic converter is the P0420 code.

2. Acceleration Lag

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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 close to the exhaust pipe end and ask someone to push the acceleration pedal to increase your RPM to 2000. If you get a meager amount of exhausts coming out, it might be a bad catalytic converter. Compare the flow to a similar car.

3. High or low fuel consumption

Bad Fuel Economy

High or low fuel economy is one of the symptoms of a poor catalytic converter. Due to improper combustion of the fuel, it reduces or increases the overall fuel efficiency of the car, resulting in increased costs. The most common reason is high fuel consumption due to the lower power of your car.

4. Sulfur Smell

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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 an obvious indication of your catalytic converter’s malfunction.

Discolored Housing

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The converter discolors’ outer surface due to overheating 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 gas flow through the converter and creates a back pressure that affects engine performance. Thermal damage can also occur when hot converters contact the cold area, damaging their parts.

5. Starting Problems

Hard Starting

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. The catalytic converter can also be so clogged that the engine cant gets the emissions out of the engine, but the engine will most often start for 2-3 seconds in this case.

6. Increased Emissions

Rough Idle

If your catalytic converter went really bad, there is a risk that the catalytic converter is not doing its job properly anymore, and this will cause an increment of the car engine’s emissions. The overall purpose of the catalytic converter is to remove emissions, so this is maybe pretty obvious.

7. Engine Misfiring

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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?

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 car’s pollutant gases into less harmful pollutants. The structure of the catalytic converter has the shape of a honeycomb.

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

Since combustion occurs 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 engine’s exhaust gas flow. A perfect converter usually lasts up to 10 years. The honeycomb structure of the catalytic converter starts to suffocate slowly, which affects the engine’s performance.

It is sometimes difficult to find out that a car’s catalytic converter is not working perfectly. A bad converter’s effects 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.

Catalytic Converter Location

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The catalytic converter is located on the exhaust pipe system, between your muffler and the exhaust pipe manifold.

Depending on the car model, it is installed at different lengths away from the exhaust pipe manifold. It’s under the hood on some cars, and on other cars, it is located almost in the middle of the car.

Catalytic Converter Replacement Cost

The average catalytic converter replacement cost is between $150 and $2000. The labor cost for the replacement is often between $50 and $400 while the catalytic converter costs $100 to $1600, depending on whether you are looking for a universal catalyst or an OEM part.

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 always to replace it with an OEM catalyst to reduce future problems with it. Catalysts are susceptible parts and must be first class.

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.