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  • A clogged catalytic converter will result in poor fuel economy and engine performance, a failed emissions test, and stalling.
  • Antifreeze and oil contamination as well as physical damage to the catalytic converter can cause it to fail prematurely.
  • Using a vacuum gauge and testing for back pressure are common ways of checking whether or not the catalytic converter is clogged.
  • There’s no way of fixing a clogged or damaged catalytic converter. You’ll need to replace it with a new one.

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 reduce these harmful emissions by using precious metals to convert toxic gasses 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. A clogged catalytic converter will present symptoms, while a chemically failed cat con that’s not clogged typically just illuminates the check engine light or malfunction indicator lamp (MIL). Also, if the cat con that just fails chemically but doesn’t clog is not the light-off cat con (the cat con closest to the engine with an O2 sensor keeping tabs on it), then that failed cat con won’t cause any symptoms at all.

A clogged catalytic converter will present symptoms, while a chemically failed cat con that’s not clogged typically just illuminates the check engine light or malfunction indicator lamp (MIL). Also, if the cat con that just fails chemically but doesn’t clog is not the light-off cat con (the cat con closest to the engine with an O2 sensor keeping tabs on it), then that failed cat con won’t cause any symptoms at all.

Richard McCuistian, ASE Certified Master Automobile Technician

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. The blockage restricts the flow of the exhaust, hindering the engine’s air intake. This leads to a wide range of engine performance problems.

Here are some common signs of a clogged catalytic converter:

Poor fuel economy and engine performance

A clogged catalytic converter disrupts the air intake of your vehicle’s engine. As a result, it reduces your car’s acceleration and overall performance. In addition, you may also notice a drop in your fuel.

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 abruptly.

A clogged catalytic converter can also keep the engine from starting.

Illuminated malfunction indicator lamp

The check engine light is your car’s way of telling you that something is wrong with it. In other words, it’s warning you that the engine is not performing optimally and that you need to examine it for damage.

Though the check engine light/ MIL may not illuminate if the catalyst is only beginning to clog, that doesn’t mean you should leave your cat be. Inspect it regularly to make sure there’s no blockage or damage.

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Failed emission test

A clogged cat can trigger the check engine light. This can cause you to fail an emissions test. Furthermore, if the clogged cat suffers internal damage, your car becomes far less likely to pass a tailpipe emissions test.

Clogs can even cause parts of the catalyst 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.

What’s even worse is that on some engines, if the “honeycomb” part of the catalyst begins to break up, particles of the cat can enter the cylinders and destroy the engine.

Other Ways of Diagnosing Catalytic Converter Failure

Using a Vacuum Gauge

One quick and easy test is to connect a vacuum gauge to a manifold vacuum port (like the one feeding the brake booster). Idling on a healthy engine, the gauge should read 18-22 inches of vacuum idling in park or neutral. Wear safety glasses and remain clear of moving parts while you have an assistant raise the engine speed (still in park or neutral). If you can operate the throttle yourself from under the hood (be careful) that works too.

On a healthy engine, the vacuum should dive very briefly and then recover to a slightly higher reading than where it was at idle. If the vacuum drops with the engine speed raised and stays lower than it was with the engine idling, the catalyst is probably clogged. If raising the engine speed a little more lowers the needle even further, it’s more certain. At this point, move to the next step below.

If you have the tools to do it (this can be tough on some vehicles), 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 or the exhaust removed, chances are high that the catalytic converter is the component that’s causing the problem. Of course, the engine is extremely noisy when the sensor is out or the exhaust is removed and if you drive the vehicle with the O2 sensor removed or the exhaust disconnected, the heat can be intense enough to cause damage to nearby components, so don’t run the engine too long or too much with the O2 removed or the exhaust disconnected.

You may also check the structure of the converter for any impact damage caused by running over road debris, but any road debris that has damaged the converter usually causes other noticeable damage.

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 (available from online suppliers for $20-$30). These kits typically require removal of the O2 sensor.

If you want a more accurate reading, you can use a digital manometer ($40-$50 online), or a pressure gauge with a variety of units of measurement. Any of these require some kind of connection to the exhaust in front of the converter being tested.

If your engine has a secondary air injection system, you can disconnect the check valve and install your pressure gauge. You may get a more accurate result if you connect the check valve to the exhaust system before the converter. But note that connecting to the check valve (which is usually a ⅝ hose connection) would require building some type of adapter).

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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. If you drill a ⅛ hole you can simply hold a square cut hose up to the hole while an assistant increases engine speed. After the test you can install a self-tapping screw or, better yet, a blind pop rivet in the ⅛ hole.

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. Less than 1 psi is optimum.

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.

, How to Tell If the Catalytic Converter is Clogged

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Be aware that the “brick” in a catalytic converter can, in some cases, break loose enough that it can intermittently turn sideways so that the backpressure problem isn’t always present. But in that case, whacking the converter with a rubber hammer will create a rattle that will pinpoint the loose brick.

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

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

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.

Actually, replacing a clogged cat is the only way to fix the problem. Removing the catalytic converter and replacing it with a pipe is a breach of federal law, even in states that don’t do emissions testing. Don’t go there.

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. Most vehicles have two converters – one near the engine (the light-off cat that handles NOx). This one is monitored for efficiency by a downstream O2 sensor. The cat that handles the HC and CO is the rearmost cat (usually a separate unit from the front one on newer cars) and is not monitored by a downstream O2.

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). A properly balanced fuel mix will produce mostly CO2 and water. A slightly rich mixture will produce CO (not enough O2 to go around) and a leaking injector or VERY rich mixture will produce high HC (hydrocarbon – unburned fuel) in the exhaust, which shows up as soot.

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NOx is produced when combustion chamber temperatures exceed 2500 degrees, because most of the atmosphere in the mix (78%) is nitrogen, which is superheated by the combustion event to drive the piston down. Above 2500 degrees, nitrogen molecules bond to O2 molecules in various different compounds – that’s why it’s referred to as NOx (x can be any one of several different numbers).

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.

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.

Watch this video to understand how catalytic converters work:

You can also check out this video for tips on how to replace your catalytic converter:

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 catalytic converter might fail, including:

  • Contamination from substances such as antifreeze and oil
  • Dents that can damage the catalysts
  • Engine performance problems

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.

Good aftermarket catalytic converters, which are available at a lower price than OEM converters, will help make sure you don’t get a new catalyst efficiency code.

Products Mentioned in this Guide

Clogged Catalytic Converters: The Final Takeaway

Symptoms of a Clogged Catalytic Converter

  • Poor engine performance and fuel efficiency
  • Engine stalls or struggles to start
  • Check engine light is lit
  • Failed emissions test

What Is a Catalytic Converter?

  • Reduces emissions through chemical reactions
  • Breaks down toxic greenhouse gasses
  • Converts nitrogen oxides into breathable nitrogen and oxygen

How Does a Catalytic Converter Work?

  • Engine produces toxic emissions and sends them out the exhaust
  • Catalyst breaks down exhaust gas to make harmful gas breathable
  • Second catalyst breaks down exhaust gas to convert more toxic substances
  • Converted breathable gas disperse through the tailpipe

What Causes a Clogged Catalytic Converter?

  • Coolant, antifreeze, and oil can contaminate the exhaust
  • Engine misfires can cause the cat to malfunction
  • Dents in the catalyst and internal damage
  • Engines that run rich or lean due to improper air-to-fuel ratios
About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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I have a 4.6 v8 with o2 sensor codes and cat codes on I’ve been driving it forever without any problem. All of a sudden I get a P0016 code which is the Vvt solenoid for bank 1. I’m just wondering if it’s possible for the cats to create enough back pressure to throw timing codes. I’ve already had the solenoid replaced and I’m not trying to spend a lot of money trying to figure the problem out. There is no power above 2k and it sounds exactly like a constant misfire without the stutter. Sounds like a whole different car while idling. Just hoping it’s not timing chain related or worn guides.

Thanks for any response I appreciate your time!

D Langon

I had my check engine light come on showing it to be either a catalytic convertor or O2 sensor are bad code come up. Brought the car into Nissan it’s a 2013 Rogue. After checking it out they said the O2 sensor tested fine. Cleared the code and could not replicate the failure after test driving it. The car drive great and still does. 3 days later the check engine light came back on. Brought it to auto zone to get a read on the light. Now it’s shows that catalytic convertor or O2 sensor again plus the exhaust manifold? Anyone have any ideas before I bring it back into Nissan and pay $2000 for repairs?


What would cause entire exhaust system to glow orange from the secondary cat all the way back to, and including the exhaust manifold? 2.0 4 cyl from a 2006 Hyundai Elantra 186K miles. Flashing CEL under load, and constant CEL with P0303 code (cylinder 3 misfire)
This car had it’s spark plug wires criss-crossed when I got it. It still ran, due to the deployment of waste-spark. When I changed to the correct order and put new plugs in, in ran great, faster than my Acura, for about a month. Then the above problems began. Any thoughts?


Sounds like it is running lean


You timing is off, that is why it ran with the plug wires crossed. I had a vehicle with straight pipes and a glass pack in place of the muffler the second half of the exhaust pipe was missing it through flames out the exhaust once I got it above 70 mph and let off the accelerator it would backfire a little and throw flames. The timing was off by 180 degrees.

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