Everything You Should Know About Catalytic Converters

Fuel, Emission and Carb

By Rodney (Mechanics)

Catalytic converters might be one of the greatest inventions that we use on our vehicles. This component works really great to reduce the emissions produced by vehicles. They work by cleaning the dangerous gases leftover from the combustion process. The result is a reduced level of tailpipe emissions such as hydrocarbons and carbon monoxide. If the vehicle is operating normally, it can even reduce the pollutant points to almost zero. But you should also know that this component might not work properly sometimes. Emissions level might rise up along with a lower engine performance that leads to the failure of an emission test for your vehicle.

Conditions Created by Problems in Converters

Under normal conditions, you should be able to use your catalytic converters for more than 150,000 miles of usage. But sometimes the catalyst will experience contamination that will make it unable to function properly, which can also lead to a higher number of afterburners and emissions. Clogging may also happen to the ceramic honeycomb due to the piling up of carbon deposits. A problem that can happen from this condition is excessive back pressure that will lower your engine performance. In a more extreme condition where the converter becomes totally plugged, your engine could also stall because it won’t be able to get rid of the exhaust gases.

You might also be able to notice some symptoms from the problems on the catalytic converters. The symptoms include a drop in your fuel economy, a lack of power on your engine while driving in normal condition, engine stalling the first time you start your vehicle and a rough idle condition. All of those symptoms might be caused by excessive back pressure from a dirty converter.

Some elements that are found in motor oil and antifreeze, such as phosphorus anti-wear additives or silicate corrosion inhibitors, can also contaminate the proper working of a converter. This will lead to the inability of your catalyst to clean up the dangerous gases. You can notice this because there will be an increase in carbon monoxide and hydrocarbon emissions. Also, when the converter efficiency becomes lower than 90%, your vehicle will also set the Check Engine Light on. The system will also record trouble codes and your vehicle will not be able to pass an emission test.

While fouling might not always cause excess back pressure on the converter, carbon can start to build up in the converter honeycomb if you don’t fix the problem quickly.

But you also need to understand that your converters will not just get dirty for no reason at all. You should always diagnose and fix the problem that is causing your converters to have carbon buildup. But, even if you can identify the problem, a dirty condition on your catalytic converter is usually just the start of your problem. Replacing a bad converter without fixing the real problem is just going to be a temporary solution. If the real cause is still untouched, then you will still need to replace your new converter in a short time.

Catalytic Converter Operation

The good news is that if you’re using a good catalytic converter, then this component will be able to reduce your tailpipe emissions, such as oxides of nitrogen, hydrocarbons, and carbon monoxide, by up to 98%.

Converter Operation

The proper catalyst converter should not have to process the emissions very hard if the vehicle is in a normal condition. The normal condition means that you have a good mechanism in compression, fuel combustion, fuel mixture, ignition system, and engine management. If all those things are working normally, then the level of carbon dioxide produced by the engine should be less than 0.01% and less than 50 PPM for the hydrocarbons.

A catalytic converter should be able to function at the light-off temperature from around 400 to 600 degrees Fahrenheit. Meanwhile, they should also work properly in the normal operating temperature from 1200 to 1600 degrees Fahrenheit. The operating temperature of your converter will usually rise up depending on the level of pollutants contained in the exhaust.

On many late model engines, you might also be able to find “Pre-Cat” or “Pup Cat” mini converters. They are usually located in the exhaust manifold. Some engines may place it inside the head pipe that is normally found between the manifold and the converter. This mini converter is used to reduce cold-start emissions when the catalytic converter is still warming up. They can heat up quickly and it will reduce the level of hydrocarbons and carbon dioxide the first time you start your vehicle. But they will not be able to reduce oxides of nitrogen because this element will only be produced when the engine is hot.

Newer engines that are equipped with fuel injection mechanisms usually have cleaner combustion. They will not require the converter to do a lot of work. You can notice this by a small difference in temperature between the inlet and outlet converter. The difference may be lower than 30 degrees Fahrenheit at 2500 RPM. On older engines, this temperature difference can reach 100 degrees Fahrenheit at cruising speed. Due to this fact, it is not always wise to determine the condition of your converter just by measuring the temperatures of the fore and aft part when the converter is idle.

But you can still check if your converter is overheating by measuring the temperature. Using an infrared pyrometer, you can measure the temperature by pointing it to the converter when your engine is running. You should check the temperature readings on the fore and aft parts of the converter. If there are different temperatures between those parts that are more than 200 degrees Fahrenheit, then that means that there is a problem with the converter. The most common reasons for this include dirty spark plugs, a bad ignition system, or a leak on the exhaust valve. This can also happen from a fuel mixture that has too little amount of air.

Lean misfires can also be the cause of a high number of tailpipe emissions. The reasons for this might be some leaks on the engine vacuum, the leaky valve on the EGR, dirty injectors, or low fuel pressure. Even one misfiring can produce an increase of more than 2500 PPM of hydrocarbons emissions. You can also notice the rising temperature of your catalytic converter.

Another symptom that can tell if your converter is running too hot is the changes of colors on the converter shell.

Causes for Plugging on the Catalytic Converter

The main part that can be damaged from the prolonged high temperatures on the converter is the honeycomb substrate. The high temperature will cause melting or partial collapse on the component. A misfiring spark plug could be the underlying cause for this case. You could also get this result if you have a burned exhaust valve. This condition will leak unwanted compression and excess fuel to get into the combustion chamber.

Exhaust Backpressure Checks

Checking to see if your catalytic converter is plugged may be done by inspecting the condition of the intake vacuum and the exhaust backpressure. To inspect the intake vacuum, you can use a vacuum gauge to get a vacuum reading in idle condition. Accelerate the engine speed to around 2,500 RPM and read the number again. The number should be about 18 to 20 inches Hg when the engine is idling. After accelerating the engine, you should notice a momentary drop in vacuum reading.

If you get a reading that is around 10% lower than the normal number, then the problem might be from the deposit buildup on the backpressure. But this condition can also arise from broken ignition timing. Some engines are also much more sensitive to vacuum changes compared to newer ones. Always make sure that you check the backpressure to get a more complete picture of your engine condition.

You can also use a pressure gauge tool to inspect the condition of backpressure. Normally, the gauge that you should use will be calibrated in 1/2-inch increments and can read up to 10 PSI. If you are using a metric measurement, remember that one PSI equals 6.895 Kilo-pascals.

There are many ways that you can connect your pressure gauge to the exhaust system. The first step is to connect the gauge to the main hole in the exhaust system after removing the oxygen sensor. Secondly, you can also connect the gauge to the air pump system after removing the air check valve. And lastly, you can connect it by drilling a small hole on the head pipe but not on the converter. The downside of drilling a hole is that you will need to cover or weld the hole completely again after you do the test. You will not be able to drill a hole if the converter is built with double-wall construction.

To start the measuring process, you should start the engine and inspect the reading of the backpressure. The number will usually depend on the type of vehicles you have. The backpressure reading should be 0 when the engine is idling. Meanwhile, when the engine is running at 2500 RPM, the number should not be more than 1.25. Other vehicles might be able to go more than that number, but the number should never be more than 4 PSI.

An extremely high number, such as 8 to 10 PSI, indicates that there is an exhaust restriction on the engine. This condition will require a further diagnosis from experts. But this number can also be caused by a collapsed pipe or broken muffler, and not just a plugged converter.

Always check the condition of your pipes and muffler to make sure that they are not the cause of your problem. You can also drill a hole in the aft part of your converter to check the level of backpressure. If the reading comes out with less than 1 PSI, then you can be sure that your engine system is okay. You can then proceed to disconnect the converter from the exhaust pipe. If you notice a drop in the back-pressure number, then the problem might be caused by a damaged condition on your pipe or muffler.

Checking to see if the converter is plugged is also very easy. You can simply disconnect the converter and shine a light from one end to the other. If you can notice the light coming through the honeycomb, then you can be sure that your converter is clean and not plugged.

When you shake the converter, you should not be able to hear a rattling sound from the inside. If you can hear a noise like that, then the honeycomb substrate has probably experienced cracking or is in damaged condition.

Converter Scan Tool Checks

You can check whether your converter is experiencing a restriction, plugging, or wearing out by using a scan tool. These are the things that you should check out for:

  • If you are using a V6, V8, or V10 engine, you can use the scan tool to find a significant difference in the readings of short-term fuel trim and long-term fuel trim. These engines will have two different cylinder banks, and if you get different levels of reading from both, it will mean that one of the converters is experiencing plugging.
  • If your engine is equipped with a Mass Airflow sensor, then it will be able to calculate a value of barometric pressure. Check the number using the scan tool. If the number is lower than normal, and there is a restriction on the exhaust system.
  • If it has a sensor to calculate the value of Manifold Absolute Pressure, then you can be sure that your engine has a restricted converter if the reading is lower than normal.
  • If the scan tool detects a lower-than-normal value for Calculated Load, then it means that your engine is experiencing a lower performance efficiency. This is because the engine cannot get enough air because there is a restriction on the exhaust system.
  • If the scan tool detects code P0420 or code P0423, then it means that the converter is not operating normally. If these codes appear, it means that every component in the engine is okay and you just need to replace your converter with a new one.

If the scan tool that you are using has a feature to read “catalyst efficiency”, you can also measure how good the converter is at reducing emissions. If the number of efficiencies is lower than 90%, it means that the condition of your converter is bad and you need to get it changed.

Causes of Catalyst Fouling

In the process of cleaning the exhaust of your vehicle, the catalytic converter will get exposed to the hot gases produced by the exhaust. Some of these elements include lead, silicone, and phosphorus. These elements are the ones that can contaminate the quality of your converter. While lead is getting rarer these days, phosphorus is still a threat that can result in contamination. Phosphorus is a byproduct of burning your motor oil. It will be able to damage the good condition of your catalytic converter. If you can notice blue smoke coming up from your exhaust, then it also means the catalytic converter has been contaminated with phosphorus.

The level of phosphorus contained in motor oil has been greatly reduced since 2005. It has helped to prolong the life of catalytic converters. This element can also be found on anti-wear additives. It has been proven to reduce the possibility of contamination on catalytic converters.

Meanwhile, silicate can get into the exhaust system if there is a leak in the combustion chamber that lets internal coolant get in. Silicate will not only contaminate the converter, but it will also affect the condition of oxygen sensors. If this condition happens, then the oxygen sensors will also require a replacement.

A symptom of internal coolant leak can be noticed by white smoke that comes out from the exhaust system.

Catalytic Converter Operating Efficiency

The Powertrain Control Module (PCM) is the component that monitors and calculates the efficiency of your catalytic converter. It does this by comparing the reading levels of upstream and downstream data from the oxygen sensor.

Little activities on the downstream oxygen sensor mean that the converter is working at high efficiency. When the converter is experiencing a loss in efficiency, the oxygen sensor will also show a higher reading number. If the efficiency number has reached a certain limit, the Check Engine Light will be turned on to notify you of a problem.

You will not be able to check the converter efficiency yourself. This will need to be done by a professional repair shop with the help of an exhaust analyzer. But this will also be difficult to do because most repair shops will usually rely on the trouble codes to analyze the problem with the catalytic converter.

OBD II Catalyst Monitor

The condition of engine sensors, emission controls, along with the overall engine performance can be diagnosed using the OBD II Diagnostics that are equipped on the vehicles that are manufactured since 1996. The check engine light will be turned on when the diagnostic tool detects a problem that could lead to an increase of 50% of emissions produced by the engine.

One part of that system is the catalyst monitor that will run when the engine is experiencing a certain condition. For instance, the monitor will be triggered when the vehicle is driven at cruising speed for at least 10 minutes. The engine should also have a normal temperature when being used.

Note: If there is a fault in the oxygen sensor, then the catalyst monitor will not run, and the function will be overridden.

When the PCM is reading data from the oxygen sensor, it will also need a reading from the catalytic converter. If the converter cannot respond quickly enough, the PCM will assume that there is a drop in the efficiency level of the catalytic converter. Normally, a trouble code of P0420 will get triggered. But other fault codes ranging from P0420 to P0439 can also occur.

Catalytic Converter Code

The main objective of using a catalytic converter to maximize the cleaning up of emissions gases.
The regulations forbid you to use a straight pipe as a replacement for a broken converter. Always replace it with a new one or use an OEM converter.

Catalytic Converter Warranty

You should always check if your catalytic converter is covered under warranty before replacing it with a new one. With a warranty, you can get a free replacement of the catalytic converter by just bringing your vehicle to your car dealer. Be informed that you may need to pay for other components that you also need to replace, such as the pipes, oxygen sensors, and other components.

In the United States, all cars are usually equipped with Federal Emissions Warranty. This regulation guarantees that catalytic converters that are only used under 8 years or 80,000 miles should be replaced for free. Some states on the East Coast region of the United States also have their own regulations for emission warranty. The warranty in these states will only cover for no more than 7 years or 70,000 miles. The warranty is about 15 years or 150,000 miles for some hybrid vehicles that are PZEV certified.

Note: Make sure to check the manufacturing date of your vehicle. This is because the warranty coverage is counted from the manufacturing date, and not the sale date. You can usually find this manufacturing date on the decal or your vehicle plate.

While there are used converters available, you should know that the warranty is usually shorter, which is about two years or under 24,000 miles. Some of these catalytic converters will also not give you a good performance compared to the original ones and may trigger trouble code P0420.

Replace Catalytic Converter

You should promptly replace the catalytic converter if it has been contaminated, plugged, damaged, or has rust on it. Also, if the trouble code is showing a low efficiency from the catalyst, the converter should also be replaced with a new one.

On some engines, the trouble code will also tell you which one of your cylinder banks needs to get its converter replaced. For instance, if the trouble code indicates that there’s something wrong with the right cylinder bank, check the converter and other components on the right part of your engine cylinder.

When you replace the catalytic converter, it is also recommended to replace the oxygen sensors. Prolonged usage of your oxygen sensor can contribute to slow response time which will lead to inefficiency in the fuel mixture. This will also lower the efficiency of your catalytic converter and some trouble codes might occur.