How to Tell If the Catalytic Converter is Clogged

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More than 90% of cars today are powered by internal combustion engines (ICE)—and until electric vehicles completely take over, this type of engine is likely here to stay. However, it isn’t ideal as it can create harmful toxic chemical compounds that have a negative environmental impact.

Catalytic converters help combat these harmful emissions by employing precious metals to convert toxic gases into non-threatening substances. Due to increasing global environmental awareness, this exhaust system component, also referred to as a “cat” for short, is considered one of the most important inventions in the history of automobiles.

catalytic converters installed in a car
Catalytic converters help combat harmful emissions by employing precious metals to convert toxic gases into non-threatening substances.

Signs of a Clogged Catalytic Converter

A clogged catalytic converter is different from a cat that is worn out or damaged chemically, although they may present some of the same symptoms. In other words: not all bad catalytic converters are clogged catalytic converters. A catalytic converter can lose its efficiency without being clogged.

But the biggest problem with a clogged cat is that it can create excessive exhaust backpressure. Basically, because the clogged cat restricts exhaust flow, it prevents the engine from “breathing” properly—which can lead to a wide range of engine performance problems.

Here are some common signs of a clogged catalytic converter:

Illuminated malfunction indicator lamp

The check engine light is your car’s way of telling you, “I need to see a doctor.” It’s basically warning you about certain issues that are causing your engine to perform poorly and inefficiently.

There are different reasons why the check engine light can illuminate. The best thing to do after seeing this warning is to get a mechanic to diagnose it for you.

Difficulty starting the engine and stalling

Excessive exhaust back pressure can choke the engine, causing it to stall. In the case of extreme blockage, you may find your engine starting and idling for a couple of minutes, only to die shortly afterward.

Poor fuel economy and engine performance

A clogged catalytic converter prevents your engine from breathing properly. As a result your car may experience a lack of acceleration and overall performance. You may also notice the fuel economy drop.

Failed emission test

A clogged cat can trigger the check engine light – and that will cause you to fail an emissions test in most locations. Furthermore, if the clogged cat has come apart inside, your car is unlikely to pass a tailpipe emissions test (if one is required).

As previously mentioned, clogs can even cause the catalyst materials to break into pieces. If you suspect that you’ve reached this level of damage, give the cat a tap and listen for rattling noises.

car emission test apparatus attached to an exhaust pipe
A clogged catalytic converter can trigger the check engine light and can cause you to fail an emissions test.

Other Ways of Diagnosing Catalytic Converter Failure

One of the first things an experienced mechanic will do to trace a clogged converter is they will temporarily remove the oxygen sensor or unbolt the exhaust downpipe. If engine performance improves with the sensor removed, chances are high that the catalytic converter is the component that’s causing the problem.

You may also check the structure of the converter for any impact damage caused by running over road debris.

Testing for Backpressure

There’s another way of confirming your suspicion—by measuring the backpressure. To do this, you’re going to need a low pressure gauge with a scale that reads up to 15 PSI. You may also opt for a basic backpressure test kit.

If you want a more accurate reading, you can use a digital manometer, or a pressure gauge with a variety of units of measurement.

If your engine has a secondary air injection system, you can disconnect the check valve and install your pressure gauge. However, you can get a more accurate result if you connect the check valve to the exhaust system before the converter.

You could also drill a hole along the exhaust pipe (just ahead of the catalytic converter) where you can attach the pressure gauge. This is your best option if you have an older car and are worried about damaging the O2 sensor.

The backpressure in an idling vehicle can vary depending on its year, make, and model. Typically, an engine at idle should have 1.5 PSI at most. Of course, some engines can go way higher, but the rule of thumb is to have 1.5 PSI and below.

When you rev the vehicle at 2,000 RPM and it remains steady at or below 3 PSI, then you likely don’t have issues in your catalytic converter. But if it fluctuates or increases beyond 3 PSI despite having a steady RPM, it’s a good indication that backpressure is building up.

Revving up the engine will increase the pressure, which is normal. However, an abnormal increase in pressure at a steady RPM can indicate possible backpressure.

Other Ways to Check Back Pressure

It’s worth noting that you can also check back pressure with a vacuum gauge (not a pressure gauge). Connecting a gauge to a source of manifold vacuum is much easier than connecting a gauge to the exhaust.

Additionally, it may be possible to get an idea of back pressure by monitoring a scan tool and looking at parameters, such as manifold absolute pressure (MAP) and calculated load.

When to Replace or Repair Your Catalytic Converter

A clogged catalytic converter should be replaced. Although there are products on the market that claim to be catalytic converter cleaners, you can’t believe everything you read.

Most professionals will tell you that replacing a clogged cat is the only way to fix the problem.

What Does the Catalytic Converter Do?

A catalytic converter reduces the emissions from the engine’s exhaust by means of a chemical reaction. If you split the assembly in half, you’ll have two primary sections.

The first one is a catalyst with a combination of platinum and rhodium. It works by breaking down oxides of nitrogen (NOx) into nitrogen and oxygen molecules.

As the exhaust gas travels further, it passes through a second catalyst, which is a combination of platinum and palladium. Here is where two-way oxidation takes place—carbon monoxide (CO) and hydrocarbons (HC) are broken down into less harmful molecules such as carbon dioxide (CO2) and water (H2O).

The oxygen (O2) molecules from the first catalytic conversion bond with CO molecules and form CO2. The hydrogen and carbon molecules from the hydrocarbon compound split to bond with oxygen and form CO2 and H2O.

emissions from a car with a catalytic converter
You won’t pass an emissions test if your catalytic converter is damaged or not working properly.

After the whole process, the now-less-harmful exhaust gas travels until it reaches the end of the tailpipe, where it’s dispersed into the atmosphere.

For your car to pass the emissions test, the catalytic converter should be in optimal condition.

What Can Go Wrong With Your Catalytic Converter?

Damaged catalytic converters not only cause your vehicle to fail an emissions test, but it can also damage neighboring parts, resulting in more expensive repairs.

There are multiple reasons why your cat might fail prematurely, including:

Foreign substances, such as coolant and oil, can get into the exhaust due to engine problems upstream. Such contaminants can easily destroy your catalytic converter.The same goes for dents, as a blockage in the catalysts can also be a result of physical damage.

Engine performance problems can damage your catalytic converter, as well. For example, an engine misfire or an improper air/fuel mixture can cause the cat to overheat. And that can lead to its early demise.

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Staff Writers

In the Garage with is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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