catalytic converter rattle

Your car’s catalytic converter is arguably the most critical component of the exhaust system. It takes toxic gasses produced in the engine that is pushed through the exhaust manifold and then breaks them down into less harmful emissions. They then pass through the muffler and the tailpipe, where they pass relatively harmlessly into the environment.

Most of the time, the catalytic converter does this with little noise or fanfare. So, when you start to hear a rattle coming from the car’s underside toward the firewall with the engine, it’s definitely a cause for concern.

You should never procrastinate a catalytic converter rattle, whether it’s caused by something seemingly minor, like a loose heat shield, or something as serious as a damaging clog.

To help you understand what’s causing your catalytic converter rattle, what it’s telling you, and what you need to do about it, we’re going to have to delve into the depths of this sophisticated exhaust system component.

What Does a Catalytic Converter Do?

The internal combustion process inside the engine block creates harmful toxic gasses that pass directly through the exhaust manifold to the catalytic converter. Here, two “Catalytic” events occur.

The first uses the metal catalysts within the converter to help reduce harmful nitrogen oxides. It essentially separates the molecules into nitrogen and oxygen, actively trapping the nitrogen within the catalytic converter while allowing the released oxygen to pass through to the muffler.

The second catalytic event uses the intense heat of the catalytic converter to transform unburned hydrocarbons left over from the internal combustion process while oxidizing the dangerous carbon monoxide.

The waste gasses are then sent to the muffler, which deadens the sound of the engine before being passed relatively harmlessly out the end of the tailpipe.

The Catalytic Converter’s Impact on Engine Performance

The exhaust manifold and the catalytic converter have a series of sensors that actively read the content of the exhaust gasses leaving the engine when it’s running. This data is then sent directly to the car’s ECU, which reads the amount of oxygen and other gasses to determine how efficiently the engine runs. Any imbalances detected are then corrected by minute changes in the fuel/air ratio or the engine’s timing.

If a problem occurs with one of these sensors or the catalytic converter, it will affect engine performance. This often triggers the ECU to turn on the check engine light and throw a code.

Many of these catalytic converter problems show early signs of rattling. Whether or not it’s a serious internal fault within the catalytic converter that needs to be repaired immediately will hinge on the source of the rattle.

Causes of Catalytic Converter Rattle

If you’re lucky, your catalytic converter rattle is simply a loose heat shield that can be easily repaired. However, more serious catalytic rattle causes, like a fracture or a clog in the ceramic interior honeycomb, are likely more expensive to repair.

1. A Loose Heat Shield

A Loose Heat Shield

The intense heat of the catalytic converter or contact with road debris can knock the heat shield loose. Sometimes rust affects a portion of the heat shield’s weld, causing it to come loose in a car that’s 7 to 10 years old. This is even more likely to happen if you live near saltwater spray.

This is a problem that often starts small and seems to pop up out of nowhere. One day, you’re accelerating, and you hear a small, strange rattle. It might only happen when you accelerate hard at first. Then, the catalytic converter starts rattling when you start the car. Then it rattles anytime the engine gets over 2,500 RPMs. This is a sign that the failure in the weld, the rust, or the fastener is progressing.

If it gets to the point where the catalytic converter is rattling intermittently while the engine is merely idling, it likely means that one whole side of the heat shield has unzipped itself. At this point, it’s only a matter of time before it completely fails.

The real concern here is that as the heat shield continues to fail, it’s letting more heat energy from the rattling catalytic converter escape. At some point, this becomes a real fire risk. The loose heat shield can also leave the catalytic converter vulnerable to damage from road debris. It can also allow water to get at the catalytic converter, increasing the risk of a more serious rust problem later on.

How to Fix a Loose Heat Shield on a Catalytic Converter

The quick fix for a rattle due to a loose catalytic converter heat shield is to resecure it with extra-large pipe clamps. All you have to do is crawl under the car when the exhaust system is cold, wrap two or three of the clamps around the catalytic converter, and tighten them down. This will usually work and stop the rattle for a few months.

Unfortunately, this is not a long-term fix. Even the highest quality stainless steel hose clamps will eventually break down and fail. The rattle will return again.

The clamps also do nothing to prevent rust and other failures in the heat shield from worsening unchecked. So, when the clamps do fail, the catalytic converter rattle will be far worse than it was before. Chances are also good that there won’t be enough rust-free metal remaining for a mechanic to weld it.

If you take the car in immediately and have the funds available, the best move is to have a mechanic repair or replace the catalytic converter’s heat shield.

The part cost for a replacement heat shield will cost around $125 to $150.

The labor cost to repair or replace the catalytic converter’s heat shield will add another $75 to $150 to the final repair bill of $200 to $300.

2. A Clogged Catalytic Converter

Clogged Catalytic Converter

When a catalytic converter is clogged by carbon deposits and other types of combustion residue, it can make a rattling sound when the exhaust gas passes through it. Depending on the severity of the clog, a variety of other symptoms may also manifest.

Along with a rattling noise when accelerating and starting the car, you’ll likely also notice sulfurous rotten egg odors coming out of the tailpipe. If the clog is severe, it will also impact overall engine performance, causing the car to feel down on power and possibly even creating wisps of black smoke in the exhaust.

The build-up of waste gasses in the exhaust manifold will eventually cause the check engine light to come on. This will throw and store one or more of the following codes.

  • Code P0420 is a general code for a catalytic converter problem. It almost always happens when a partial or severe clog affects the performance of the exhaust manifold and, thus, the engine. It’s usually detected by the downstream oxygen sensor, which measures the gas leaving the catalytic converter.

The good news is that a lot of automakers set the tolerance level for Code P0420 when they see a 10% decrease in efficiency or less. So, if you heed it right away, the rattle-inducing clog in the catalytic converter might be minor.

  • Code P0162 is the diagnostic code for a problem detected by the upstream oxygen sensor that is placed between the exhaust manifold and the catalytic converter. When this code is triggered, it’s a sign that the catalytic converter is so clogged or damaged that waste gasses are building up in the exhaust manifold.

How to Fix a Clogged Catalytic Converter Rattle

Suppose you catch a clogged catalytic converter early, and you only get an error code P0420. In that case, you might be able to clear it by pouring a catalytic converter cleaning additive into the fuel tank. Then, you would run the car on a full tank of gas, and hopefully, it would clear the clog.

If the catalytic converter cleaner additive doesn’t work, or you’re getting a P0162 error code indicating a severe clog that’s causing a backup of waste gasses in the exhaust manifold. At this point, you’ll need a professional mechanic to step in.

They might be able to physically clean the catalytic convert at a cost ranging from $350 to $550. Though most likely, they’ll need to replace the rattling catalytic converter. Especially if the car is more than ten years old.

Note

Unless your car is more than ten years old, chances are good that the catalytic converter didn’t start clogging up on its own. There’s probably some other fault affecting the engine performance. The carbon buildup and other deposits in the catalytic converter are just secondary symptoms.

Unless your car is more than ten years old, chances are good that the catalytic converter didn’t start clogging up on its own. There’s probably some other fault affecting the engine performance. The carbon buildup and other deposits in the catalytic converter are just secondary symptoms.

The most common causes are:

  • A blown head gasket
  • A cracked cylinder head
  • Oil leak
  • Bad piston rings
  • Failing valve seals
  • A stuck-open fuel injector
  • A severely clogged fuel filter

3. Damaged Internal Honeycomb

Damaged Internal Honeycomb

If the ceramic honeycomb structure inside your catalytic converter is cracked, damaged, or partially collapsed, it can create a throaty rattle. You often notice it most when you accelerate hard or first start the engine.

The honeycomb can crack with age and time, especially if you live somewhere with four extreme seasons. Though all it might take is accidentally grounding out on a large speed bump or a road obstacle to crack the honeycomb.

When the honeycomb inside a catalytic converter is damaged, it will also affect its ability to trap and process harmful emissions. You’ll likely notice strange sulfur odors, like rotten eggs, coming from the exhaust. The rattle coming from the catalytic converter will be sharper and happen more often, the worse the damage is.

Unfortunately, a catalytic converter with a damaged honeycomb can rarely be fixed. The catalytic converter will need to be replaced entirely.

How Much Does It Cost to Replace a Catalytic Converter?

The cost to have a mechanic replace a rattling catalytic converter that’s failed due to a damaged internal honeycomb or severe clog can vary wildly.

If you have a slightly older car, you might be able to get away with a universal catalytic converter. This is designed to be a general fit for a wide range of vehicles. They’re engineered to work well enough to let you pass an emissions test but aren’t ideal for a high-performance engine.

The part cost for a universal catalytic converter is usually around $250 to $350.

A muffler shop technician can usually cut your old catalytic converter off and weld the universal one on for another $125 to $200.

If you have a newer vehicle or a high-performance tuned engine, then you’ll need a “Direct Fit” catalytic converter. It’s specific to the make and model of your vehicle, which means it will have a higher part cost.

The replacement direct-fit catalytic converter can range wildly, but you can expect an average of around $850 to $1,000.You might also have to wait for the part to come in, or you’ll have to pay for it on special order.

The cost to have a muffler shop install it will add another $125 to $400 to the final repair bill.

Frequently Asked Questions

Will a rattling catalytic converter fail an emissions test?

If the rattling sound is due to a loose catalytic converter heat shield, and the attendant doesn’t hear the sound, your car might still pass the emission tests. If the catalytic converter is rattling due to a partial clog or a damaged internal honeycomb, the decrease in efficiency will likely be so severe that you easily fail the emissions test.

Does an emissions test include a physical inspection of the exhaust system?

An emissions test is technically meant to analyze your car’s exhaust into the atmosphere. In some states, this doesn’t require any sort of physical inspection of the exhaust system itself. However, if an emissions tester hears a rattle or any other sort of sign of an exhaust system problem, they will likely scrutinize it. So, something as seemingly minor as a loose heat shield might possibly cause you to fail an emission test, even if the rest of the catalytic converter is operating normally.

Conclusion

The most likely reasons for a rattling catalytic converter are a loose heat shield, a clog, or a damaged internal honeycomb. If it’s a loose heat shield, you might be able to fix it yourself with some stainless steel pipe clamps quickly. However, this is only buying you 6 to 9 months before you need a muffler shop to replace the original heat shield.

If you get a check engine light with a P0420 error code, your catalytic converter rattle might be due to a partial clog. If you catch it early, some catalytic converter cleaner additive in the fuel tank might clear up the problem. Just be sure to look for other possible engine faults that caused the clog!

If the rattling catalytic converter is due to a severe clog or a damaged internal honeycomb structure, then you’ll most likely need to replace the catalytic converter. If you have a slightly older car, you might be able to get away with a universal catalytic converter for less than $600. Otherwise, a direct-fit catalytic converter can easily cost you more than $1,000.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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