Clogged catalytic converter: How to test and replace

Learn how to test for a clogged catalytic converter and what to do if you have one

Despite their robustness, catalytic converters can encounter several issues that may impair their functionality, from simply setting a check engine light to causing a no-start condition or even stalling the engine as you drive. If you suspect you’ve got a clogged catalytic converter, I’ll show you how to test it and what to do if the test shows it’s clogged.

Learn the symptoms of a clogged catalytic converter

• Reduced engine performance and power— You notice that you have to press farther down on the accelerator to get the same pickup as before.
• Poor fuel economy— A clogged converter robs the engine of efficiency, so fuel economy drops
• Difficulty starting the engine— Because the converter is clogged,

Melted and plugged catalytic converter

the engine experiences too much exhaust backpressure, and can’t breathe properly
• Misfiring or rough idling— Exhaust backpressure causes cylinder misfires and a rough idle
• Rotten egg smell from the exhaust— The portion of the converter that’s still working is overloaded and can’t properly break down the exhaust gasses
• Check engine light illumination— A clogged converter will fail the catalytic converter efficiency test and will set a P0420 or P0430 trouble code
• Excessive heat under the vehicle— Excessive fuel or oil in the exhaust can cause the converter to heat beyond its 1,200°F operating range, causing the metal casing to turn blue and the structure inside to melt.

How to test for a clogged catalytic converter

Buy or rent an inexpensive vacuum gauge from any auto parts store. Connect the rubber hose from the gauge to any port on the intake manifold. Then,

Buy or rent a manifold vacuum gauge to test for a bad converter

1) Run the engine at idle. The vacuum reading should remain steady and within specifications. If vacuum drops toward zero, that in an indication of a plugged catalytic converter.

2. Slowly increase engine speed to 2,000 rpm. The vacuum reading should remain as high as when idle.

3. Quickly close the throttle. The vacuum reading should rise rapidly to above idle level. If the reading is 5 inHg or more below normal and then returns slowly and unevenly, that indicates a plugged catalytic converter.

What to do if you have a clogged catalytic converter

A clogged catalytic converter is a dead catalytic converter. There’s no way to bring it back from the dead.

Look at the images below. They show what a clogged catalytic converter look like once its removed from the vehicle. The ceramic structure is permanently damaged, clogged, and melted. There are no chemicals that can restore a clogged converter back into operating condition.

Inside view of catalytic converter with melted ceramic structure

Before you shop for a new converter, find out what killed the old one

Catalytic converters don’t die on their own. There is always an underlying reason for its failure. In the case of a clogged catalytic converter, the clogging is usually due to a severe overheating condition that causes the ceramic structure to melt. Cylinder misfires caused by rich air/fuel mixtures are the most common cause of overheating, followed by internal oil or coolant leaks.

If you don’t diagnose and fix the underlying cause, you will damage the new converter and be back in the same spot within months.

How to shop for a new catalytic converter

When selecting a replacement catalytic converter, several key factors should be considered to ensure you choose the right one for your vehicle.

• Compatibility—Ensure the replacement is designated to work on your car and is the right one for the year, make, model, and engine in your vehicle. There’s no wiggle room here. It has to handle the volume of exhaust from your particular engine.

• Type— There are two options for replacement converters: direct-fit and universal.

Direct-fit converters match the exact specifications of your original converter. They bolt up or weld to the exhaust manifold or existing pipes without any modifications, making installation much easier.

While universal converters are often less expensive, they often require exhaust system modifications, adapters, additional support hangers, and welding. Choose a universal converter only if you have the necessary tools and expertise or access to professional installation services.

• Warranty— An aftermarket (non-dealer) catalytic converter warranty varies depending on the manufacturer and the specific product. However, many aftermarket catalytic converters come with a warranty period that ranges from one to five years. Some manufacturers might also offer warranties based on mileage, commonly around 25,000 to 50,000 miles.

Where to buy a replacement converter

If you aren’t installing the converter yourself, a locally-owned muffler shop is the best place to shop for a new one. Muffler shops deal with converter replacements daily, whereas a mechanical shop may replace one catalytic converter per month. Muffler shops know which brands of converters work best with which cars, and they’re generally less expensive than a mechanical shop.

However, as mentioned earlier, a replacement catalytic converter won’t last long if you haven’t fixed the underlying problem. Ask the muffler shop what the warranty is on the replacement converter, but understand that the warranty won’t cover failure caused by an engine problem. The same applies if you buy a replacement converter and a mechanical shop.

©, 2014 Rick Muscoplat