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A Simple Way To Reveal A Clogged Exhaust System

Do you suspect that you have a clogged exhaust system? You have probably found an overwhelming amount of info on how to diagnose this possibility. And by now, it looks too complicated or too expensive to diagnose on your own. Don’t worry, there is a simple way.

Clogged Exhaust System

By now, you have probably read on how to use an expensive OBD2 scanner, or an exhaust gas analyzer. Possibly check the temps from the exhaust manifolds to the rear pipe. All of this isn’t bad advice, but it can get expensive and really complicated. However, I have a cheaper and much simpler method.

The Answer to Diagnosing a Clogged Exhaust System

So, I am sure you are wanting to know the simple method to diagnosing a clogged exhaust system. So, here is the quick and shot answer. Use a vacuum gauge.

I will go through some of the methods that are out there, mostly the ones that make sense. However, going back to basics and old school with a vacuum gauge is by far the easiest method.

I will get into the details on how to use the vacuum gauge to identify a clogged exhaust system shortly. Just know that a vacuum gauge is far cheaper than scanners and gas analysis tools. Plus, it really takes the guesswork out of it.

The trick is knowing how to read the gauge and what you are looking for. Like I said, I will get into the details further into this article.

Using an OBD2 Scanner

I am sure you have found tons of information on how to use an OBD2 scanner to determine clogged exhaust. However, most of this information is really complicated and not totally conclusive.

Even though a scanner can tell you exhaust temperatures and the O2 outputs. It also can tell you emission outputs, and with this info combined. You might be able to determine if you have a clogged exhaust system. But in reality, you are still just taking a guess.

O2 sensors can give false readings based off becoming defective, or the engine running too rich. There are many factors that can cause scanner readings to not completely indicate a specific issue.

Some car manufacturers onboard computer will generate an error code. This code might indicate a clogged exhaust system. But often more than not, clogged exhaust will not generate a code.

A more expensive OBD2 scanner might have a better chance at identifying a clogged exhaust. But you are talking at least a $500 scanner. The more expensive scanners are bidirectional. This means it can control certain functions that might help eliminate components until you find the issue.

If you use a scanner, look for bad and or failing emission readings and test. Look for high exhaust temperature readings. Look for irregular O2 sensor readings. Compare the readings between the upstream and downstream O2 sensors.

Using an Exhaust Gas Analyzer Tester

clogged exhaust system

You would think, to identify a clogged exhaust system with a gas analyzer would be the best way. However, you will again get readings that can be a factor from a bad running engine due to a failing component.

Gas Analyzers are expensive to start off with. It would be cheaper to go to a muffler shop or a garage. Let them run a diagnostic with a gas analyzer if you really feel strongly about using one.

A gas analyzer measures the levels of you O2, CO2, HC, Co and NOx. But much like your OBD2 scanner, you will just get emission level readings. This is still not going to be an exact science on whether the exhaust system is clogged or not.

A clogged exhaust system or clogged catalytic converter can cause rich readings. However, this is just telling you that your engine isn’t burning the fuel correctly. It’s a lot of money to pay for inconclusive information.

Bad injectors, a vacuum leak, bad O2 sensors and more can cause your engine to run rich. These types of issues, often do not generate codes, much the same as a clogged exhaust system doesn’t.

You can read more on Gas Analyzing Readings on this page by Walker Exhaust Systems.

Testing for Backpressure

Some engines EGR Systems have tubes and lines that can be tapped into and attach a pressure gauge. You can start the engine and see how much back pressure you have coming from the exhaust.

Overall, an engine at idle should never have more than 1PSI. Revving the engine up past 2500 RPMs, it should never exceed 3PSI. However, if you feel your readings are high, you can start trying to pin point the blockage.

If you are lucky and your vehicle has several O2 sensors along the exhaust system. You can replace the sensors with an adapter to plug your gauge onto.

This sounds great in theory, but some vehicles like Subaru, make the header pipe and converter together. In other words, the pipe/manifold that come off the engine merges into a converter. This makes testing for a clogged exhaust system more difficult.

clogged exhaust system

Subaru’s main converter has the O2 sensors in it. Both the upstream and downstream are close together. Testing pressures at these two points reveals very little.

Subaru is also known for having a rear and second converter and that one doesn’t have any sensors to tap off of.

Many muffler shops will drill into the exhaust pipe at various points and screw in a self-threading adapter. This way, you can keep testing till you get to the rear of the exhaust to see where the pressure drops off.

This kind of test can help determine what is clogged within the exhaust. You can have clogged catalytic converters, muffles or even a pipe itself.

Checking for Temperature Differences

With the engine running and fully warmed up, you can use an Infrared Thermometer to identify exhaust blockage. A catalytic converter should never really reach a temperature more than 1200°F. Nominal should be between 500°F and 800°F.

When measuring the exhaust temperatures, it is best to aim the thermometer at the welds. You will notice that where the exhaust is put together, there is ring of weld. Aim your thermometer at these points.

You can start at your engine manifolds or header pipes to get a general starting point. Then aim the thermometer at the first weld ring, which is usually at the catalytic converter. Check that one, then compare to the second weld ring.

This test can be done all the way down the pipe including mufflers and resonators. If the exhaust isn’t clogged, there shouldn’t be more than a 20°F difference throughout the system.

If you are getting temperature readings higher than 1200°F, this can be an indicator that component is clogged. High temperature variances throughout the exhaust pipe, is also an indicator of a clogged exhaust system.

This task can also be difficult to do for the at home mechanic. Unless you have lift, laying on your back trying to preform these test can prove to be difficult.

Using a Vacuum Gauge to Test for a Clogged Exhaust System

clogged exhaust system

Out of all these methods, this is by far the easiest and quickest way to tell if you have a clogged exhaust system. The hardest part of using a vacuum gauge is finding a place to hook up to on the engine.

As the cars get newer, the more commonsense diagnostic ports they remove. So, I can tell you, the fastest way to hook up to vacuum. is to the brake booster. Usually, this line is a 3/8 hose. You will need an adapter or a “T” fitting.

Once you have hooked up your Vacuum Guage, you will need to monitor the readings. Start the engine and note where the vacuum is. It should be pulling somewhere between 16 and 22 inches of vacuum on a start.

As it idles, the vacuum will decrease, and the needle of the gauge may become unsteady and shaky. Any time you rev the engine up, the vacuum will drop to zero, this is normal. However, if the exhaust is clogged, the vacuum will not come back to what it was when you started it.

When an exhaust system is clogged. The exhaust backpressure will start backing up into the combustion chamber and into the intake. Thus, causing a loss of vacuum and making the gauge needle fluctuate.

Watch this video and see how this Subaru went from 18 inches of vacuum down to 5 inches just idling. It had two clogged catalytic converters and you can see the low vacuum and how the needle was unsteady.

2013 Subaru 2.5L – How To Diagnose/Replace Clogged Catalytic Converters

This Subaru baffled several garages because everyone was using their OBD2 scanners. I decided to go old school with the vacuum gauge and I figured it out.

Now to test which parts were clogged, I dropped the section with the second converter in it. However, it really didn’t help it. Although, the Subaru 2.5L main catalytic converter is made into the header pipes. I ordered a new one and started it with the second converter still disconnected.

The car revved up like a race car, and the vacuum help steady at 19 inches of vacuum. When I hooked up the second catalytic converter, it went back to a shaky vacuum drop and wouldn’t run correctly.

The second catalytic converter had loose material in it, you could hear it rattling around inside. I took the pipe off and poured out chunks of the honeycomb grid and a ton of powder.

I cut out the second converter and replaced it with a flex pipe and hooked it back up. The engine ran great and the vacuum held steady at 19 inches of vacuum and never dropped.

This indicated to me that the muffler was still in good shape.

A Few More Signs of a Clogged Exhaust System

With the car running, go to the exhaust pipe and feel the exhaust coming out. If there isn’t much flow of air blowing out but intense heat. This is a sign of a clogged exhaust system.

Have someone rev up the car, while you feel with your hand. If you do not feel a good thrust of exhaust come out, that is a good sign it is clogged.

The exhaust system may have a strong smell of sulfur or rotten eggs. If so, this is a strong sign the catalytic converter is getting clogged up. You may also notice dark exhaust smoke coming from the tailpipe.

Your vehicle may have a loss of power or poor gas mileage. You may also notice extreme heat coming out from under the vehicle. These are also signs of a clogged exhaust system.


Take notice of how your car is running, do a comparison from month to month. If you feel it is losing power or using more gas, you should have your exhaust checked.

The way newer exhaust systems are made and the materials used to make them. Most exhaust systems don’t really try to clog until around 125000 miles. That doesn’t mean they can’t clog up before then.

The type of fuel you use, how often you change your air filter, to the environment you live in. These factors can make a difference of how fast your exhaust can get clogged.

There is a preventative method to help keep your catalytic converters clean. Use a Catalytic Converter Cleaner additive about 4 times a year.  These cleaners will help your exhaust from getting build up which will eventually clog the system.

What makes this diagnostics challenging, is when the computer will not generate codes. However, if you are lucky enough to get a code, you can review the OBD2 Definitions on RodsShop.

Even though I strongly believe in using a vacuum gauge. Having a nice OBD2 scanner can help with other issues. Look below, to discover a far cheaper way to replace using a $500 scanner. TOAD is a system that you can plug into your laptop and have all the feature the big shops have.


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