The Diagnosing Bad Catalytic Converters – Clogged Catalytic Converters. Catalytic converters are the engine’s emissions system’s heart. However, due to their internal structure, they are also the most restrictive part of the exhaust system. Catalytic converters, also known as ‘cats,’ generate backpressure and restrict exhaust air flow. Despite their negative impact on performance, they are critical in keeping exhaust gases clean.
When catalytic converters fail, it is usually due to clogging. When the cats become clogged, the exhaust gases are further restricted and are unable to exit the exhaust system. This increases backpressure, which sends exhaust gases back into the combustion chamber, causing a variety of performance issues and causing your car to fail emissions inspection.
This guide will explain what catalytic converters are, the symptoms of a faulty or clogged catalytic converter, and how to diagnose and repair a faulty catalytic converter.
What Exactly Is A Catalytic Converter?
Catalytic converters are components of a combustion engine’s exhaust system. On turbocharged vehicles, the cats are typically part of the downpipe, which is the first pipe in the exhaust system and connects directly to the turbocharger. They are usually installed after the headers or exhaust manifold and before the muffler system on naturally aspirated vehicles, hence the name “cat-back” exhaust system.
The number of cats in a vehicle varies. Most cars with true dual exhaust systems, including twin turbocharged models, will have two cats. Most cars have both primary and secondary catalytic converters, which are sometimes located within the same exhaust piping. Every car has at least one catalytic converter, and some have as many as four.
Having cats in your car is a legal requirement in all 50 states in the United States because the cats are responsible for cleaning exhaust gases and making them more environmentally friendly.
Catalytic Converters: How Do They Work?
A catalytic converter’s interior is a honeycomb structure made of rare earth metals such as platinum, palladium, and rhodium. The primary byproduct of exhaust is nitrogen oxide, which is extremely harmful to the environment. When nitrogen oxide (NOx) collides with these metals, a chemical reaction occurs that breaks down the NOx into carbon dioxide and water.
The primary catalytic converter is typically made of platinum and rhodium. It converts NOx into pure nitrogen and oxygen molecules. These molecules are then passed through a secondary cat made of platinum and palladium. When molecules reach the secondary cat, chemical oxidation occurs, converting carbon monoxide and hydrocarbons into carbon dioxide and water, which eventually exit the exhaust tailpipes.
Finally, catalytic converters use rare metals to initiate chemical reactions that convert harmful exhaust gases into less harmful gases.
What Causes Catalytic Converter Failure?
Metal is a major component of a cat’s internal structure. The exhaust gases that exit the engine are extremely hot. Cylinder misfires and other ignition-related issues can result in unburned fuel entering the exhaust system. The unburned fuel then ignites in the catalytic converter when this occurs. When the metals melt, they close up the honeycomb air chambers and clog the exhaust system because they are in a honeycomb structure.
When a catalytic converter becomes clogged, it creates backpressure, allowing exhaust gases to enter the combustion chamber. This, in turn, raises exhaust gas temperatures, further melting and clogging the cats. It can also cause misfires, which send more fuel into the exhaust system.
While clogged catalytic converters are the most common cause of failure, they can also fail in a variety of other ways. The honeycomb structure inside the cat can completely melt away, leaving the cat hollow. When a cat completely hollows out, there are usually no symptoms other than a check engine light for a bad O2 sensor reading. However, your car will still fail emissions because the exhaust air is not properly cleaned.
Catalytic Converter Clog Symptoms
- Misfires in the cylinders
- Idling of the engine is rough.
- Increased exhaust gas or engine temperatures
- Engine light on (O2 sensor or air-to-fuel ratios)
- Inadequate fuel economy
- Emissions test failure
As previously stated, symptoms of bad catalytic converts are most likely to appear when your cat is clogged and not completely burned through. Outside of a check engine light, there will be fewer symptoms if it completely burns.
Furthermore, if your catalytic converter is badly clogged, you will notice performance issues such as loss of power, lack of acceleration, and hesitation while accelerating.
1) Cylinder Failures
When a cat clogs, exhaust gases are returned to the combustion chamber. This extra air throws off the air-to-fuel ratios, which can eventually lead to cylinder misfires. These misfires can then clog the cat further by sending unburned fuel into the cat.
2) Uneven Engine Idling
Rough idling is another possible symptom of increased backpressure. Misfires or fuel pockets burning in the wrong order can cause rough idling.
Too much backpressure from a clogged cat can even cause the car to stall while idling. This is usually caused by far too much air in the combustion chamber to ignite the fuel.
3) Enhanced EGTs
You may notice an increase in temperatures if you have an exhaust gas temp gauge (EGT) in your car. Exhaust gas is extremely hot, and recycling it back into the cylinder can raise engine temperatures and, as a result, exhaust gas temperatures.
4) O2 Sensors / Check Engine Light
Failed cats can also cause the O2 sensors to fail. Furthermore, the O2 sensors are in charge of monitoring the efficiency of the cats, so a failing cat will result in a check engine light for bad O2 sensor readings.
Misfires or lean air-to-fuel ratios can also cause a check engine light to illuminate.
5) Inadequate Fuel Economy
The ignition system will send more fuel into the combustion chamber to compensate for more air in the chamber. This will result in increased fuel consumption and lower gas mileage. However, you are unlikely to notice more than a 1-2mpg decrease, making this symptom difficult to detect unless you meticulously track your mileage and driving habits.
6) Negative Emissions Test Results
Finally, a faulty catalytic converter will result in a failed emissions test. However, this can also be affected by the type of emissions test you receive. If you’re not throwing any engine codes, you might be able to pass an OBD-II test. You will, however, fail a tail-pipe test.
Overall, driving with a faulty catalytic converter is illegal, so whether or not you pass emissions, it should be replaced. You may also notice foul-smelling exhaust gases coming from your tailpipe, which may alert cops to a bad cat or a lack of cats.
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How to Identify a Faulty Catalytic Converter
Unfortunately, the symptoms of a faulty catalytic converter are very similar to those of a variety of other engine problems. Aside from a rattling noise and a bad exhaust smell, there are two ways to diagnose a bad catalytic converter.
- Vacuum Gauge Examination
- Take out the O2 Sensors
- Backpressure Examination
Vacuum Gauge Examination
Vacuum is produced by all combustion engines. Remember in middle school science class when they taught you how to siphon water? Vacuuming is similar, but it uses air. Engines suck air in through the intake due to the vacuum pressure created by the engine.
A vacuum gauge is required to perform a vacuum test on the cat. The vacuum gauge must be connected to a vacuum port. Most vehicles have one that feeds the brake booster, which is a good place to start testing. Connect the vacuum gauge and start the engine. At idle, check the vacuum reading before increasing the RPMs. The vacuum gauge should initially drop and then rise to a higher reading than when at idle.
If you increase the RPMs above idle and the vacuum gauge drops below idle, you most likely have a clogged cat.
Removal of the O2 Sensor
The easiest way to test is to remove the O2 sensor (or sensors). Air cannot escape quickly enough when a cat is clogged. When an O2 sensor is removed, the exhaust gas has another place to escape. If you remove the sensors and the misfires, rough idling, and other symptoms go away, the problem is most likely a clogged catalytic converter.
If the symptoms persist, it may indicate a different engine problem. However, because a failing catalytic converter may not show any symptoms, this method is not always effective. In general, it will work if your cat is clogged, but it will not work if you’ve burned through all of your cats.
Backpressure Examination
I hope we’ve hammered home the fact that a clogged catalytic converter causes backpressure. As a result, testing the backpressure is a good way to see if it’s bad. This will require the use of a low pressure tester as well as the removal of the O2 sensors. The O2 sensor bung will be connected to the low pressure tester. If you have more than one O2 sensor, make sure you connect it to the one ahead of the cat.
Start the engine and measure the backpressure at idle. While the backpressure at idle varies, it should be less than 1.5 psi. The lower the number, the better. Check your engine’s backpressure every 1,000 RPMs. Backpressure should rise as RPMs rise, but it should be 3 psi or lower at 2,000 RPMs. If your backpressure rises significantly when you rev it, you most likely have a clogged cat.
Catalytic Converter Replacement Options
Of course, the most straightforward option is to replace your catalytic converter with an OEM unit. However, this is a costly option. Cats, as previously stated, use rare metals within their bodies. Palladium, platinum, and rhodium are not inexpensive metals. As a result, cats are not cheap. Nowadays, stealing catalytic converters is more popular than stealing cars because they are so valuable and nearly impossible to track once stolen.
When it comes to replacing catalytic converters, most people choose one of two options.
- Catalytic converters with high flow rates
- Catless pipes, also known as “test pipes,”
Catalytic Converter Replacement with High Flow
Catalytic converters are limiting. The best exhaust system for a car, especially a turbocharged or supercharged car, is none at all. As a result, many performance companies offer cat replacements for power gains.
The first option is to use high-flow catalytic converters, also known as turbo downpipes. Because high flow pipes contain cats, they are legal in the majority of states. They are less restrictive than OEM downpipes because the honeycomb structure has larger cross-sections or air passages.
These are an excellent choice for anyone looking to keep their car legal while gaining a little extra horsepower. Furthermore, because they contain fewer rare metals, they are less expensive than OEM replacements.
Pipes without a catalytic converter
The second option is to remove the catalytic converter entirely. This will cause your car to fail emissions testing, usually resulting in a check engine light, and is illegal in all states.
However, due to the performance gains obtained, removing the cats is common among performance enthusiasts. The catalytic converter is removed and replaced with a standard exhaust pipe that is completely hollow, resulting in zero restriction. As a result, it virtually eliminates all backpressure. Power gains of 5whp are likely for naturally aspirated vehicles. However, the gains for turbocharged vehicles can be closer to 20-25whp.
Overall, this is an excellent choice for performance enthusiasts. It does, however, come with the risk of your car being crushed.
