How to Fix a Rattling Catalytic Converter: 4 Possible Solutions

Hearing a rattle from underneath the hood of your car is never pleasant, but it’s a common occurrence if the vehicle has driven over 100,000 miles. The rattling noise almost always comes from the catalytic converter and can usually be fixed by cleaning it. But what if that doesn’t work?

Here’s how to fix a rattling catalytic converter:

  1. Clean the catalytic converter
  2. Fix the heat shield
  3. Use high-quality fuel
  4. Replace the catalytic converter

This article will detail all of these methods and discuss other important information regarding rattling catalytic converters.

1. Clean the Catalytic Converter

The most common culprit of a rattling catalytic converter is dirt and residue built up over years of not being cleaned. The dirt can block proper airflow, resulting in poor performance and noise.

Luckily, you can diagnose a clogged catalytic converter by the smell coming from the exhaust. If it smells like rotten eggs, the converter isn’t converting the hydrogen sulfide to sulfur dioxide.

To clean the converter, you’ll need to take it out. After that, there are two methods you can use:

With Sodium Hydroxide

To remove the hydrogen sulfide, you can use sodium hydroxide.

Here are the steps:

  1. Pour the cleaning solution into a spray bottle and spray both sides of the converter over a bucket or drip basin.
  2. Let the sodium hydroxide soak for 20 to 30 minutes.
  3. After that, clean the inside of the converter thoroughly with water.
  4. You can use a hose or pour the water until the flow is clear.
  5. Reinstall the converter.

Check out this video to see how it’s done:

With Lacquer Thinner

Lacquer thinner can also be very effective in cleaning a catalytic converter. It could be handy when you notice poor engine performance and decreased fuel efficiency.

Here are the steps for this method:

  1. Add one gallon of lacquer thinner to an empty gas tank.
  2. Add around 10 gallons of gas to it.
  3. Head for the highway and drive for about 150 miles or 2-2.5 hours.
  4. The engine needs to work at 2500 RPM for at least one-third of the trip, which is why it’s best to do it on a highway.
  5. Once the lacquer thinner flows through the whole system, it should remove blockages in the converter and improve performance (the rattling should be gone too).

If the rattling persists, there might be a cracked or broken ceramic monolith. If that’s the case, you’ll need to take the car in to be seen by a pro.

2. Fix the Heat Shield

The heat shield of the catalytic converter protects the cab floor and surroundings from the enormous heat generated inside, which can reach up to 1,600°F.

With a blockage, the temperature can go over 2,000°F. This amount of heat can melt the floor carpet and rubber mats.

The heat shield can also cause rattling if it has missing or loose bolts. Stones can also get between it, causing even more rattling when coming into contact with the stainless steel.

You’ll need to remove the heat shield by unplugging the bolts to address this. You may need to use penetrating oil to get all the bolts out since the high heat may have caused them to become stubborn. It would be best to replace the bolts entirely.

Once the shield is out, use soap and water to wash the inside thoroughly. You can also use sandpaper to polish the inside if it’s particularly hazy.

Finally, reattach the heat shield by using anti-seize on the bolts. Check the torque specifications to ensure that the bolts are correctly installed and in place.

3. Use High-Quality Fuel

It may sound too simple to be accurate, but sometimes all you need to do to fix the rattling noise is use higher-quality fuel. If you’ve been using the lowest-octane gas available, try switching to high-octane and have it run through the vehicle.

This solution may be just enough to clear up airflow and stop the rattling. You can also add additives to your gas tank specifically made to clean up the cat.

For example, this OXICAT Oxygen Sensor & Catalytic Converter Cleaner can clean up the entire exhaust system if used regularly. It’s also safe to use and works with both petrol and diesel vehicles.

4. Replace the Catalytic Converter

Catalytic converters are made to last a lifetime, but outside factors, such as clogging, may cause overheating that can permanently damage the cat.

If all else fails, the only option you may have left is to replace the old catalytic converter. This may not be what you want to hear, but as with everything else, catalytic converters have a certain life expectancy, and there’s little you can do if they reach it.

Replacing the converter is also the most expensive option and typically costs from $800 to $1,200, depending on the make and model of your car. Generally speaking, the larger the engine is, the more expensive the catalytic converter.

If you have even a little bit of technical car knowledge, you can try installing the new converter yourself. However, it would be best to get a direct fit catalytic converter if you want to do so because a universal fit requires molding. In contrast, you can install a direct fit converter with bolts.

Depending on the car model, there are many ways to replace a catalytic converter. Still, the most common location is on the underside of the vehicle, which means you’ll have to do the following steps below:

  1. Raise the car and secure it on jack stands. Make sure to raise it enough to have space to move around. Don’t forget to secure the tires with stoppers or wood blocks too.
  2. Find the catalytic converter. It’s usually located toward the vehicle’s front half and has a recognizable shape.
  3. Remove oxygen sensors that may be installed on or near the catalytic converter. You can skip this step if there are no oxygen sensors.
  4. Spray penetrating oil over flanges and hardware. That’ll loosen up the nuts and bolts, making removing them easier.
  5. Be careful with tools. Before you start removing the converter, you have to remember to be mindful of how you use your wrenches and other tools. Exhaust hardware can be very rusty, and you have to make sure you don’t strip parts of it off.
  6. Replace the catalytic converter. After removing all bolts, you can carefully remove the old converter and install the new one. Always research the emission requirements for your state, as having the wrong converter installed may cause your vehicle to fail inspection.

Here is an excellent YouTube video explaining how to replace a catalytic converter:

Other Symptoms of a Bad Catalytic Converter

Before you can try fixing any issue you may have with your catalytic converter; you have to be able to recognize that there is a problem in the first place.

Here are some of the most common symptoms of a bad converter:

Poor Engine Performance

One of the first things you’ll experience when the converter isn’t working is reduced engine performance. This can come in various forms, including poor acceleration and even the vehicle not starting.

If the converter is leaking or is clogged, the exhaust can’t flow freely, which will cause the performance of the vehicle to drop. A rattling exhaust is a sure sign that something is wrong, and if left unaddressed for long, the converter may ignite the engine and permanently damage it.

Check Engine Warning Light Turns On

This is perhaps the easiest way to notice something is wrong, as the warning will be right in front of you. However, the check engine light may also appear for problems unrelated to the catalytic converter, so an OBD2 scanner would be convenient to have to find out what exactly is wrong.

Also, keep in mind that some older cars made before 2008 may not have a check engine light.

If your car does have one, I recommend that you get the OBDScar Scanner. It’s a universal scanner that’s affordable and easy to use.

Lower Gas Mileage

Along with reduced engine performance, a clogged or damaged converter system can also cause a drop in gas mileage. In addition, the reduced airflow from the congested system can cause your car to burn extra fuel, which has a domino effect as it clogs the system even more.

If you notice that your number of gas station visits has increased, you should check to see if everything is OK with the converter or engine.

Rotten Egg Smell

Sulfuric smell, which is very similar to rotten eggs coming from the exhaust, is a sure sign of a bad cat. With a clogged converter, the sulfates that make up the fuel can build up, which will make their smell detectable.

The bad smell is usually accompanied by dark smoke coming from the exhaust pipe.

Car Fails Emissions Test

A failing catalytic converter will be unable to clean the exhaust properly, and there’s a high possibility that this will result in a failed car emissions test.

The test also checks information from oxygen sensors and will detect codes from the check engine light that has been activated because of a failing converter.

What Causes Catalytic Converter Rattles

Catalytic converters don’t fail on their own, so there’s always an underlying reason that you can discover for any issues they may have.

There are three leading causes for a failing catalytic converter:

  • Structural damage – mainly from road debris or reckless driving. You’ll usually be able to identify structural damage by inspecting the converter shield.
  • Overheated/broken converters – this usually happens when the converters are clogged or there are unusual levels of O2, HC, or CO entering the exhaust.
  • Oil-coated substrate – this occurs when the converter fills up with compounds that are damaging to surfaces, preventing it from functioning correctly. The oil-fouled substrate can occur due to:
    • Using the wrong additives or fuels
    • Carbon buildup in the exhaust
    • Internal leaks.

FAQs About Catalytic Converters

How Long Do Catalytic Converters Last?

Catalytic converters are made to last for a very long time, sometimes even the vehicle’s entire lifecycle. However, depending on your driving habits, fuel quality, environment, and other factors, it can also go bad after around ten years or 100,000 miles.

Regular service and proper engine maintenance will significantly extend the catalytic converter’s life. Proper and regular cleaning is also a big help.

Can You Drive With a Bad Catalytic Converter?

It is possible to drive with a bad catalytic converter as the rattling won’t cause harm to the engine. However, it’s bad for the environment, and it can quickly become a distraction to the driver. Also, not addressing the issue can lead to other problems like fines or failed pollution control tests.

Also, while a rattling converter alone can’t damage the engine, if the cause of rattling is blockages, this can, in fact, cause engine damage, especially after you drive without fixing it for a long time.

How Much Does It Cost to Buy a New Catalytic Converter?

Since emission standard laws are becoming very strict, having a bad catalytic converter isn’t an option, so you’ll need to buy a new one if you can’t fix it. Unfortunately, while repairs typically cost around the $100 range, replacing the converter entirely will be much more expensive.

It could cost between $1,000 and over $2,500 to buy a new catalytic converter. This will depend on the make and model of the car, how old the vehicle is, and how well the owners maintain it. Specialty cars will be much more expensive to find replacement parts for.

The converter itself won’t be the only cost, as you’ll have to factor in costs for labor, oxygen sensors, diagnostics, and more. So before taking on a replacement project, make sure that it won’t cost you more than the car’s worth.

Key Takeaways

Car maintenance is crucial, but it can get pricey. And sometimes, no matter how careful you are behind the wheel, things can break or degrade to the point they need cleaning or replacing.

If you notice a rattling in or around your catalytic converter, chances are you just need to clean it. However, it could also be an issue with the heat shield, which will involve removing and replacing a couple of parts.

In more severe cases, you may need to replace the cat altogether, which, unfortunately, could be expensive.

Related: How to Make Your Exhaust Louder

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