Ever wonder what a catalytic converter is? It’s a part of an automobile exhaust system that uses a catalyst to convert toxic exhaust gasses into ones that are less toxic. So, if your catalytic converter rattles, ticks, pings, or knocks, there are different ways you can make it quiet.
If your catalytic converter is noisy, check the converter’s heat shield and tighten any loose bolts or parts. Try switching to a high-quality high-octane fuel for a while. If the noise persists, try cleaning it. If nothing fixes the noise, consider replacing it.
In this article, we’ll look at the parts of a catalytic converter, what different sounds mean and their causes, and how to tell if the converter is failing. We’ll give you seven tips to quiet catalytic converter noise, some maintenance tips, and tell you how long a converter will last, plus explain what happens when one fails and the replacement costs.
Contents (Jump to Topic)
- Parts of a Catalytic Converter
- What Does a Bad Catalytic Converter Sound Like?
- What Are the Causes of Catalytic Converter Rattle?
- Symptoms of a Bad or Failing Catalytic Converter
- How to Quiet a Rattling Catalytic Converter
- How Long Should Catalytic Converters Last?
- What Happens When the Catalytic Converter Goes Bad?
- Can You Drive a Car With a Bad Catalytic Converter?
- How Much Does It Cost to Replace a Catalytic Converter?
- Catalytic Converter Maintenance Tips
Parts of a Catalytic Converter
Catalytic converters often referred to simply as ‘cat’, convert toxic carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO2) in the exhaust to less toxic carbon dioxide (CO2), water (H2O), and nitrogen (N2).
There are many styles and shapes of converters, which fall into three basic types.
The two-way, which was used prior to 1980 primarily removed CO and HC. The three-way has been used since 1980 and removes CO, HC, and N02. The third type is the more modern three-way plus, also known as a ‘dual bed’. It injects air ahead of a secondary bed to remove more pollutants.
All mount near the exhaust manifold and ahead of the muffler. The gasses travel out the exhaust manifold through a pipe into the intake end of the converter, which is clamped, bolted, or welded in place. As the exhaust enters the converter or just ahead of it, a Lambda probe or oxygen sensor measures how much oxygen is in the exhaust gasses.
The gasses then enter a rough-surfaced heat-resistant ceramic monolith or honeycombed bed. The cells of the monolith are coated with a catalyst of platinum, rhodium, and palladium. The catalyst forces a reaction between the gasses and oxygen, converting the gasses to less toxic ones.
In the ‘plus’ types, one or two air injection tubes introduce air from the air pump into a void between the two ceramic beds. The oxygen in the air combines with gasses that haven’t been converted by the catalysts in the first bed and catalysts in the second ceramic honeycomb convert them.
The cleaned exhaust then exits through the outlet which is clamped or welded to the rest of the exhaust system. A second or rear oxygen sensor or probe is located at the outlet or further along the connecting pipe to the muffler. It relays oxygen levels to the onboard engine computer system.
The catalytic converter is protected in a stainless-steel housing or body lined with an expanding mat that protects, seals, and insulates the ceramic monoliths. A two-piece heatshield wraps the stainless-steel housing for additional protection. The heat shield reduces heat transfer into the cab and prevents dry grass or leaves from catching fire as you drive over them.
The last component of the catalytic converter system is the engine control module (ECM). When the engine is operating, the ECM compares the information from the upstream oxygen probe and the downstream probe using a mathematical algorithm. If the readings are outside the acceptable parameters, an orange warning ‘Check Engine’ lights up.
What Does a Bad Catalytic Converter Sound Like?
A catalytic converter theoretically should last a long time. However, residue from overly rich fuel mixtures can coat and damage the inside of the honeycomb. The converter can become clogged, or sections may break apart or even collapse. A dirty or failing catalytic converter often produces different noises that indicate a problem.
- Ticking Noise. A ticking noise could be caused by oil, fuel, or even coolant being burned off by the converter. The noise may occur on cold startup, when idling, or during acceleration, and can also be caused by a leak in the converter.
- Pinging Sound. Pinging from the converter may indicate some clogging due to overheating, fuel contamination, or age.
- Knocking Noise. A knocking coming from the cat could also indicate a leak, or be caused by a loose heat shield.
- Popping Noise. A popping noise when accelerating often indicates a clog in the catalytic converter that is restricting exhaust flow and causing backpressure.
- Rattling Noise. A rattle coming from below your feet often indicates a problem with the converter. If it occurs at idle or startup there’s a good chance the ceramic honeycomb has begun to fail. You may hear the noise for a minute or two and then it stops, only to start again as you apply a little gas. However, it could also be a loose heat shield, so check that first.
- Noise When Accelerating. A rattle that gets more pronounced as the vehicle accelerates usually means the ceramic honeycomb has broken loose or into pieces.
What Are the Causes of Catalytic Converter Rattle?
A catalytic converter can be damaged by overly rich fuel mixtures, leaked oil or coolant being burned, or through the use of subgrade fuel. Age and how the vehicle is driven can also lead to damage. Damage or age can cause the honeycomb structure or heatshield to rattle.
Cracked Ceramic Honeycomb Structure
The ceramic honeycomb can be damaged by impact, or due to blockage within the passages. The catalysts can become coated with residue causing blockages or even fused together by excessive heat. Heat and blockage can cause the ceramic to crack or break apart, or even disintegrate. The cracked segments can rattle and result in other engine and performance issues.
Heat Shield Becomes Loose
The rattle from a loose heat shield is fairly common. The bolts on the heat shield can become loose, corroded, or fail, allowing the shield to move around and rattle.
Symptoms of a Bad or Failing Catalytic Converter
There are a number of symptoms that indicate your catalytic converter is failing or has failed. Some are more noticeable than others and individually identify a bad converter, while others collectively identify that there is a catalytic problem.
1. Reduced Engine Performance
A converter that leaks or is clogged or blocked can make it difficult for the engine to expel exhaust, adversely affecting the engine performance. You may notice acceleration delays or stutters when starting from a standstill. Performance may decline when the engine is being made to work going up hills or inclines, it may struggle or have less power. Another performance issue is when accelerating to pass, there may be delays, stutters, or even a loss of power.
Engine performance issues are not only frustrating, they can be downright dangerous. A quick way to check if it is the catalytic converter causing the problem is to test the exhaust force and flow. With the vehicle in park, have a helper push the accelerator down so the engine runs around 2000 rpm.
Place your hand near the tailpipe to feel and observe the amount, force, and heat of the exhaust. If it seems hot, meager, or light, it’s probably a bad converter. You can also compare the amount and force with a similar vehicle too.
2. Decreased Gas Mileage
Decreased gas mileage is another symptom of a failing catalytic converter. It can result from reduced engine performance or combustion issues. Fuel efficiency drops as the engine struggles to expel exhaust through a blocked or damaged converter. You may notice the increase in the cost to travel similar distances due to greater fuel consumption.
Additionally, the engine may also have difficulty starting due to a failing converter which causes the engine to get excess fuel in the chamber. The engine may start and run for a couple of seconds, but since the gasses can’t escape fast enough, they ignite in the exhaust system, causing back pressure and the engine to stall out.
3. Rattling Sound
Another symptom of a failing catalytic converter is a rattling noise under the driver’s feet. The chambers in the ceramic honeycomb monolith can become coated with residue from rich fuel mixtures or leaking oil and coolant. The residue can build up causing blockages or the ceramic to crack, break, or collapse.
The cracked or broken ceramic pieces rattle and are initially more noticeable at engine start up. The noise becomes louder over time and can be heard when idling or accelerating. It can lead to poor engine performance and mileage, smelly and darker exhaust, misfires, and the ‘Check Engine’ light coming on.
4. Sulfur/Rotten Egg Smell from Exhaust
A bi-product of using sulfur-containing gas is hydrogen sulfide. The catalytic converter changes hydrogen sulfide, which smells like rotten eggs, into odorless sulfur dioxide. So, if you whiff the smell of rotten eggs in your vehicle’s exhaust, you know the converter isn’t doing its job.
You may also notice the smell, or that the exhaust is darker and has more odor. Exhaust exiting from a converter that works properly won’t have much color or smell. If you start to see or smell unpleasant fumes it’s a good idea to have the converter checked.
5. Illuminated Check Engine Light
The Lambda probe or oxygen sensors ahead and behind the catalytic converter relay information to the EMC which uses a mathematical algorithm to compare the amount of oxygen in the exhaust. If the results aren’t in the acceptable parameters, the ‘Check Engine’ light illuminates. If you aren’t aware of any reasons for the engine light to come on, this may be the reason.
If your vehicle has no other symptoms of catalytic converter failure, the culprit may be a dirty sensor or two. You can either clean them or have your mechanic do so. If the light doesn’t go out, it may need to be reset.
6. Increased Emissions
As a catalytic converter fails, it is less able to clean the exhaust. An increase or darkening of the exhaust is another indicator that the converter may not be working properly. If your jurisdiction carries out emissions testing, there’s a good possibility your vehicle will fail if the cat isn’t working properly.
The EMC also stores information from the oxygen sensors. Diagnostics done during an emissions test will pick up trouble codes if the ‘Check Engine’ light has been activated due to a converter failure or readings by the oxygen sensors being outside the acceptable parameters. An automotive specialist can do an inspection and report on maintenance or necessary repairs.
7. Engine Misfires
Engine misfire can be caused by back pressure from a failing converter, and also lead to damage to the converter. They are often caused by clogged converters which can restrict the oxygen flow required for combustion within the engine. Improper combustion can coat the sparkplugs with residue, causing them to misfire.
The blocked exhaust flow can heat engine chamber gasses and cause them to ignite independent of the sparkplug in the form of a misfire. A misfire or repeated misfires can affect fuel efficiency and damage the engine. A skilled mechanic should check out the vehicle before driving it too far.
How to Quiet a Rattling Catalytic Converter
A rattling catalytic converter is a warning something isn’t right with your automobile. Whether it’s caused by a loose heat shield, buildup in the ceramic honeycomb monolith, or fracturing of the monolith, there are different ways to fix the rattle. If you’re looking to address the noise or being proactive and addressing the possibility before it occurs, here are seven possible ways to quiet a rattling catalytic converter.
1. Use High-Quality High Octane Fuel
Many people drive short commutes which don’t heat up the engine or converter enough to burn off residue or carbon buildup. A catalytic converter is designed to operate within specific temperature parameters, similar to the engine. In the old days, if an engine had carbon buildup, you’d take it out and run it on the highway to burn off the carbon.
Preventing residue buildup in the converter is similar, so making your engine work harder for a couple of hours several times a month or more is good preventative medicine. Before making your engine work, it’s a good practice to fill the tank with high-quality high-octane fuel. We don’t mean topping up the tank, we mean it should be almost empty before fueling.
Quality high-octane fuel is more expensive, but it’s cheaper than a new converter, and you only have to do it every third or fourth fill-up depending on your commute. Some fuels have additives that help remove and reduce residue buildup in the engine and converter.
If you don’t have time for an extended run on the highway, the high-octane will just take longer to dissolve the residue and unclog the converter. You may find the better fuel provides better mileage and reduces other engine noise and repairs too.
2. Clean the Catalytic Converter With Sodium Hydroxide
If your car or truck exhaust smells like rotten eggs, it means the converter isn’t converting the hydrogen sulfide to odorless sulfur dioxide. The residue buildup can clog chambers and cause a rattle. If you’re mechanically inclined or know someone who is, remove the converter and clean it.
Sodium hydroxide will remove hydrogen sulfide. Pour it into a spray bottle and squirt it into both ends of the converter so it generously coats the inside honeycomb. Do the cleaning over a drip basin or bucket. Let the sodium hydroxide work for at least 20 to 30 minutes.
Once the sodium hydroxide has done its job, thoroughly clean the inside with water. Use a hose with or without a nozzle to clean out the chemicals or pour water in from both ends until the flow is clear. Reinstall the converter and clean up the mess.
3. Use Lacquer Thinner
If you’re noticing a rattle and a decrease in engine performance and fuel efficiency, there’s a good chance there’s residue buildup in the converter. An inexpensive solution is to add lacquer thinner to your fuel tank to clean the converter.
To an almost empty, or empty gas tank, pour in a gallon of lacquer thinner and then pump in 10 gallons of gasoline. Head out onto the highway for a 2-1/2-hour cruise, or about 150 miles. The engine needs to work at 2500 rpm or higher for 50 to 70 miles (30 to 40 minutes) of that distance.
The engine workout and cleaning should remove residue blockage in the converter and engine, so you should notice an improvement in performance and efficiency. Additionally, the rattle should be gone too. If the rattle isn’t gone, then it probably wasn’t caused by a just residue blockage but due to a cracked or broken ceramic monolith.
4. Fix Loose Catalytic Converter Heat Shield
The catalytic converter’s heat shield protects the cab floor and surrounding ground cover from potential combustion by the 1,200°F to 1,600°F heat generated in the converter. A blockage can cause the temperatures to run as hot as 2,000°F, which is enough to melt floor carpet and rubber mats if the shield falls off.
If the shield becomes loose, it will rattle and can fall off or even get stones trapped between it and the stainless-steel housing to produce more noise. It is best to tighten or replace the bolts if possible. You may need to use penetrating oil to loosen the bolts, but take care not to break the metal flange where the bolts go through.
Removing the bolts allows you to clean the inside face of the shield so it is shiny and reflective, and to do the same with the stainless-steel converter housing. High-grit sandpaper can help brighten the metal too. Bluing of the metal is often a sign of excessive heat which can signify a blockage in the converter chambers.
Reattach the shield and use anti-seize on the bolts. Check the torque settings so the bolts are properly tightened. If the welded bolt flanges have broken off, you may be able to find a replacement heat shield. Alternatively, use a galvanized strap clamp to secure it in place.
5. Cataclean Fuel and Exhaust System CleanerSome fuels have additives that help remove and reduce residue buildup in the engine and converter, or you can use Cataclean Fuel and Exhaust Cleaner. It’s EPA-approved and will decrease carbon buildup and other residues from the cylinder heads, tappets, fuel injectors, oxygen sensors, and catalytic converter. Removing carbon and residues will improve engine performance, fuel efficiency, and converter rattle caused by residue blockage.
Cataclean is legal in all 50 states and can be used in gasoline or diesel engines, and even hybrid ones, but not in 2-stroke engines or those using a gas-oil blend. When your vehicle is down to about 4 gallons of fuel, add a 16oz bottle of Cataclean to the tank. Just peel off the seal and pour it in. Go for a 15 minute or more drive to get the cleaner into the engine and converter.
If the ‘Check Engine’ is illuminated or the onboard computer displays error codes, clear them after the drive. Fill the tank 3/4 or more with fresh gas and drive it for 50 miles or more, especially if there were error codes or the ‘Check Engine’ was lit up.
Fuel efficiency and engine performance should improve, and rattle caused by residue blockage should cease. For the best results, add Cataclean every 3 months.
6. Sea Foam SS14 Cleaner and LubeSea Foam SS14 is a 12oz aerosol foam registered with the EPA. It removes carbon and other deposits from valves, pistons, combustion chambers, catalytic converters, and oxygen sensors to improve performance, efficiency, and emissions. The foam can be used in 2 and 4 stroke gasoline engines with direct injection, standard injection, and carburetors, but not air induction diesel engines.
For fuel-injected engines, run until the intake is warm and then shut the engine off. Remove the intake boot from the throttle and place the Hook Guide and Cleaning Tube into the throttle body. Ensure the end of the Tube is about 1/4″ from the throttle plate.
Replace the boot over the throttle body and Hook Guide. Have a helper set the parking brake and start the engine. Leaving the vehicle in park, rev it up and hold steady at 2000rpm.
Attach the tube to the spray canister and squirt until empty – about 7 minutes. When the canister is empty, shut the engine off, remove the boot, Hook, and Tube, and then reattach the boot. Let the engine sit for 10 minutes, and then restart and run for 10 more minutes, revving the engine periodically to burn off any stubborn residue.
For carbureted engines, run until the intake is warm and then turn off. Remove the air filter and restart the engine. Place the end of the injector Tube in the carburetor throat and squirt 2 to 4oz in short bursts to avoid stalling, followed by a long burst before turning the engine off. Let sit for 10 minutes, and then run for another 10 or more minutes.
Sea Foam SS14 can also be used for engine fogging too. Use of the foam should improve emissions, performance, and mileage, plus quiet converter rattle caused by residue buildup. If the rattle, performance, and emissions problems persist, the catalytic converter may be suffering from greater damage than residue blockage.
7. Replace a Catalytic Converter
Catalytic converter rattle caused by fractured or broken honeycomb monolith should be replaced before expensive engine damage occurs. Although there are aftermarket and OEE converters available, it is recommended that the replacement be an OEM product.
Always make sure the new converter matches the old one, and the emission standards for your vehicle. With tools, some skills, and time it is possible to replace a converter yourself or have a shop do it for you.
Ground to chassis clearance is often limited, so safely raise, block, and support the vehicle. Let the exhaust system cool before working on it. Spray penetrating oil on nuts and bolts that will need to be removed and to the threads of the oxygen sensors, and let sit for 4 to 12 hours.
Wearing safety goggles or other eye protection, move under and disconnect the wires from the oxygen sensors. Either remove the two sensors while the converter is still attached or wait until it is removed. Unbolt the heat shield and remove, and then unfasten clamps, bolts, fasteners, and hangers that hold the converter in place. You may want to support the converter if it is large so it doesn’t hang and damage other components, or fall on your head.
Converters connected with gasket flanges may need to be gently pried apart and the gasket removed with the converter. Those that are welded into place need to be cut free. If parts break off or are stubborn, you may need to transfer the replacement job to a professional. If everything works well, clean the connecting flanges and gasket surfaces, and prepare to install the replacement.
Apply anti-seize to the oxygen sensor threads, hand tighten them into place, and then torque them to the correct specifications. Place the gasket into the flange between the manifold and converter, and loosely secure the connection bolts. Again, support the converter if it’s heavy.
Repeat the gasket process at the exhaust pipe end of the converter. Install the heat shield and then fully tighten and torque all bolts to the specs. Replace the oxygen sensor wires and lower the vehicle to the ground.
With the car back on the ground, clear diagnostic error codes. Turn on the engine and let idle for a minute or two, and then rev slightly to get to its normal operating temp. Once warmed, let it idle for 5 minutes before increasing to 2500 rpm for 2 minutes before shutting off and letting it cool. There should be no more catalytic converter rattle.
How Long Should Catalytic Converters Last?
Depending on driving habits, the environment, fuel, and maintenance, catalytic converters can easily last 10 years, 100,000 miles, or as long as the vehicle. The longer and hotter the converter runs, the better for it and the engine.
Good maintenance practices and engine care means being proactive at preventative maintenance and addressing problems before they become major expenses. Maintenance and environment often go hand in hand, cleaning winter salts off the underside helps reduce corrosion of nuts, bolts, and other metal parts.
What Happens When the Catalytic Converter Goes Bad?
A catalytic converter can fail in several ways. It can fail due to residue coating the catalysts, which can lead to emissions problems, fines, and mandated repairs to make your vehicle roadworthy. The residue can also reduce fuel efficiency and create blockages in the converter chambers.
A blocked converter can generate temperatures up to 2000°F, which is well above the operating range. The heat can boil transmission fluid, damage the transmission, make the clutch slip, heat up the passenger cab, and even melt rubber and carpet floor mats. Additionally, a failing converter can cause back pressure that decreases power and acceleration, makes the engine stall or backfire, and otherwise damages the engine.
Can You Drive a Car With a Bad Catalytic Converter?
Driving a vehicle with a converter that has residue-coated catalysts isn’t good for the environment but it isn’t going to cause engine damage – although it’s a symptom that all is not right under the hood. It can also mean a failed pollution control or smog test and fines. In some jurisdictions, a failed emissions test can automatically sideline your vehicle until it is repaired.
If the catalytic converter has blockages that generate backpressure, that isn’t good. Blockages can make the engine run rough and lead to decreased acceleration and power. They can also cause backfires, stalls, or starting problems. The longer you operate a vehicle with a blocked converter, the greater the potential for costly engine damage, or even an accident
How Much Does It Cost to Replace a Catalytic Converter?
The cost to replace a catalytic converter depends on the year, make, model, and engine size of the vehicle, and if it is a two-way or three-way converter. Other cost control factors include whether you do the work yourself or have a professional it and if the replacement is an OEM part or a universal fit.
Depending on where you reside, the replacement may need to meet Federal EAP emission standards or California Air Resource Board (CARB) standards, which also affect the price.
An OEM or Direct-Fit is specific to the make, model, and year and often easier to install, unfortunately, they’re usually more expensive too. A universal catalytic converter is a generic model that could be made anywhere and to questionable standards. It is usually less expensive but may require more work and time to fit and install.
A third option is a recycled or used converter from an auto parts reseller. As long as the part hasn’t been damaged, it’s a less expensive option for an OEM part.
Having a professional replace the catalytic converter usually takes one to two hours or more and will cost $800 to $2,000 or more. Doing the replacement yourself will save between $60 and $145 an hour in labor charges, but means you need the tools, parts, and time to do the job. Some replacement kits come with all the parts and gaskets, others don’t. So, make sure you have everything you need before starting, especially if you don’t have another vehicle to go get missing parts with.
A 2019 F150 OEM converter will cost anywhere from $470 to more than $1,100, while a universal can cost half that amount. A 2015 Toyota Camry will set you back $275 for an OEM that meets Federal standards, or $430 for one that meets CARB requirements. A universal converter for a 2015 Camry will cost between $75 to $300. Expect to pay more for converters for older and rarer vehicles due to scarcity.
Catalytic Converter Maintenance Tips
Short commutes or those with lots of stops and starts can coat the honeycomb chambers over time with unburned residue. The residue can build up and cause emissions problems or blockages leading to engine problems and damage. The converter needs to reach its operating temperature to work effectively.
Taking longer drives a couple of times a month where the converter gets up to peak temperatures not only clears residue off the catalysts but out of the engine too. Pouring a bottle of Cataclean or similar product in your gas tank and running it through the engine every 1000 miles will also help clean out residue buildup in the converter and engine.
Here are some other suggestions:
Regular Engine Inspections and Tune-Ups
Regular vehicle maintenance, oil changes, air filter changes, and inspections are preventative strategies that protect your investment. Inspecting for oil and coolant leaks that can damage the engine and cause residue buildup on the catalyst is important too. If the ‘Check Engine’ light comes on, get the car in for an inspection, don’t wait for more problems to develop.
Inspect the exhaust manifold and pipe leading to the converter regularly to check for cracks and pinholes. They can allow air to leak into the exhaust and enrich the oxygen mixture, causing excessive temperatures in the converter. Higher heat can damage the catalytic monolith and result in expensive repairs to the drivetrain and engine.
Use High Octane Fuel
Low-octane or poor-quality fuel can damage the converter and engine. Using high-quality, high-octane fuel regularly or at least every third or fourth fill-up is another way to maintain the catalytic converter.
The higher octane helps remove residue buildup, improving fuel efficiency, quiet converter rattle, and can extend the life of your vehicle. Fuels with additives and detergents will also assist in maintaining the converter and engine.
A catalytic converter rattle is a warning sign all is not well under your hold and car. If it’s caused by a loose heat shield, tighten the bolts. Residue buildup rattle can be reduced using additives, cleaners, or high-octane fuel.
However, rattle caused by a blockage or broken ceramic monolith usually means replacing the converter before it can cause further engine damage. Hopefully, you better understand the purpose of the catalytic converter, how to maintain it, and when to replace it.