What would cause a catalytic converter to fail?

A catalytic converter is an emissions control device that reduces toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually used with both gasoline and diesel engines.

The catalytic converter was first introduced in the USA market by General Motors in 1975 to comply with tightening EPA regulations on auto exhaust. By the 1980s, most gasoline driven vehicles were equipped with catalytic converters. Today, motor vehicle emissions contribute less than half of all manmade sources of air pollution in developed countries thanks to the widespread adoption of catalytic converters.

However, like any automotive component, catalytic converters can fail over time leading to a decrease in performance and excess vehicle emissions. This article will examine the leading causes of catalytic converter failure and steps that can be taken to prevent converter failure.

What is a catalytic converter and how does it work?

A catalytic converter is a stainless steel box mounted on the exhaust pipe between the engine and muffler. Inside the converter are a ceramic honeycomb structure coated with precious metal catalysts such as platinum, palladium and rhodium.

Pollutant In Pollutant Out
Carbon Monoxide (CO) Carbon Dioxide (CO2)
Hydrocarbons (HC) Carbon Dioxide (CO2) and Water (H2O)
Nitrogen Oxides (NOx) Nitrogen (N2) and Oxygen (O2)

As exhaust gases pass through the converter at high temperatures, the catalyst facilitates chemical reactions that convert harmful pollutants into less toxic gases. Specifically, carbon monoxide is oxidized into carbon dioxide, hydrocarbons are broken down into carbon dioxide and water, and nitrogen oxides are split into nitrogen and oxygen.

Common causes of catalytic converter failure

There are several common causes that can lead to the failure or reduced efficiency of a catalytic converter:

Catalyst contamination

The precious metal catalysts inside a catalytic converter can become contaminated by fuel additives, oil ash, and other substances. This can coat the catalyst surface inhibiting the chemical reactions needed to reduce emissions. Catalyst contamination occurs most often when there is an issue elsewhere in the vehicle causing unburned fuel or oil to reach the converter. Causes include:

– Engine misfires – Combustion of air and fuel is not happening properly in the engine cylinders, sending unburnt fuel into the exhaust system and coating the catalyst. Common causes of misfires are faulty spark plugs, bad wiring, or issues with the fuel injection system.

– Oil burning – Excess oil is being burnt in the combustion chambers due to worn piston rings, valve guide seals, or other engine issues. This oil ash can coat the catalyst.

– Fuel injectors – Leaky or dirty fuel injectors can cause unmetered fuel to reach the converter.

– Running out of gas – The fuel tank being allowed to run dry causes the fuel pump to draw in air which will reach the converter.

– Use of leaded gasoline in a catalytic converter equipped vehicle. Lead contaminates the catalyst.

Thermal damage

The ceramic honeycomb structure inside the catalytic converter is designed to operate at very high temperatures generally between 800 – 1000°F. However, it cannot withstand prolonged exposure to excess exhaust temperatures beyond this range without losing its structural integrity and efficiency. Causes include:

– Upstream exhaust leaks – Leaks in the exhaust manifold or piping leading to the converter allows extremely hot exhaust gases to hit its front face.

– Engine timing issues – Incorrect valve timing causes misfires, engine run-on, and overheating.

– Use of wrong spark plugs – Incorrect heat range spark plugs for the engine can cause overheating.

– Overheating – Any issues that causes the engine to run hotter than normal operation such as low coolant, stuck thermostat, faulty cooling fan can lead to converter overheating.

Catalyst breakdown

The catalyst has a finite life span and will eventually cease to work effectively after prolonged use. The average catalytic converter will function properly for 80,000 to 120,000 miles before needing replacement. Factors that can shorten catalyst life include:

– Use of gasoline containing sulfur – Sulfur contaminates the catalyst decreasing its lifespan. Modern US market fuels have very low sulfur content.

– Driving habits – Short trip driving and excessive idling does not get the converter hot enough for efficient operation allowing deposits to build up on the catalyst.

– Original equipment quality – Aftermarket and salvaged yard converters tend to have lower quality catalyst materials and construction compared to OEM converters.

Physical damage

The ceramic honeycomb structure inside the converter is fragile and can crack due to physical damage:

– Impact damage – Hitting debris in the road, speed bumps taken at excess speed, bottoming out the vehicle.

– Vibration damage – Loose engine mounts, damaged engine or transmission mounts can cause excessive vibration leading to cracked substrates.

– Converter theft – Some vehicles have had their converters stolen by thieves looking to extract the precious metals inside. This damage requires replacement.

Melted substrate

The ceramic honeycomb substrate can melt if exposed to very high sustained temperatures generally above 1300°F. Causes include:

– Engine modifications – Modifications causing higher exhaust temperatures such as turbochargers, superchargers, computer tuning chips.

– Reduced exhaust backpressure – Removal of the muffler and catalytic converter simultaneously causes very high exhaust temperatures.

– Engine fire – Any engine fire that spreads to the converter can melt the ceramic substrate.

Symptoms of a failing catalytic converter

Drivers will notice certain symptoms indicating their catalytic converter is no longer functioning properly:

– Check Engine Light – An illuminated Check Engine Light is very often caused by a faulty catalytic converter or related failure. The on-board diagnostics system detects emissions levels are too high and sets a trouble code.

– Rotten egg odor – A distinct sulfurous smell from the exhaust indicates the converter is no longer processing pollutants properly.

– Reduced performance – As converter efficiency fades, there is increased exhaust backpressure impeding engine performance. Acceleration will suffer.

– Poor fuel economy – Inefficient catalytic conversion contributes to reduced fuel mileage. Expect a 10-20% drop in MPG.

– Rattling noise – Internal damage to the ceramic honeycomb can cause a loud rattling sound, especially during acceleration.

Preventing catalytic converter failure

Proper maintenance and driving habits can maximize the lifespan of your vehicle’s catalytic converter:

– Maintain regular oil changes – Keep oil changed at manufacturer specified intervals to avoid oil contamination reaching the converter.

– Address Check Engine lights promptly – Diagnose and repair driveability problems like misfires that can contaminate the catalyst.

– Follow fuel octane recommendations – Use the minimum octane rating recommended in your owner’s manual for proper combustion.

– Drive gently and avoid heavy loads – Aggressive acceleration and hauling heavy loads causes converter overheating.

– Warm up the engine before driving – Allow the engine to reach normal operating temperature before driving to avoid catalyst damage from raw, unburned fuel.

– Avoid engine mods – Altering air fuel ratios and exhaust backpressure can lead to converter failure.

Diagnosing catalytic converter failure

There are a few methods professional technicians use to diagnose catalytic converter problems:

– Visual inspection – Removes the converter for visual signs of external damage like cracks, punctures or rust-through. Also inspects if the internal substrate has collapsed which impedes exhaust flow.

– Backpressure testing – Measures exhaust backpressure upstream and downstream of the converter. A working converter will have a noticeable pressure drop. A failed converter will have nearly equal pressures.

– Temperature testing – Uses an infrared thermometer or temperature probe to check the inlet and outlet temperatures of the converter. A healthy converter will show a temperature drop indicating catalytic activity is taking place.

– Gas analysis – A 5-gas exhaust analyzer is inserted before and after the converter to measure carbon monoxide, hydrocarbons and oxygen levels. A working converter will show a reduction in CO and HC levels after the converter.

– Efficiency testing – Uses a NOx gas analyzer to determine the percentage of nitrogen oxides the converter is reducing. Anything less than 80% efficiency indicates failure.

Replacing a catalytic converter

Once diagnosed as faulty, the catalytic converter needs to be replaced to avoid excess emissions and pass an emissions inspection. There are a few options to choose from:

– OEM units – Opt for a Genuine Toyota, Honda, etc. converter from the dealership matching the exact unit that came on the vehicle from the factory. Most reliable option but very pricey.

– Aftermarket converters – Aftermarket converters from trusted brands like Bosal, Walker, Eastern Catalytic can offer similar performance and durability at a lower cost. Make sure to get the proper CARB rating.

– Used OEM converters – Lightly used OEM converters removed from wrecked vehicles can be a cost-effective option. Ensure there is no damage before installing.

– Salvage yard converters – Cheapest option but unlikely to match OEM performance and durability.

The catalytic converter is bolted onto the exhaust system and a new one can generally be installed in under an hour by a competent DIY mechanic. Professional installation at a muffler shop may be worthwhile for those without the tools, lift access, or willingness to take on the job. Expect 1-2 hours labor charges for installation.


Catalytic converters play an essential role in reducing the environmental impact of internal combustion vehicles. But like any component they are subject to eventual failure. Being aware of the common causes of catalytic converter failure and following practical steps to prevent them, can help reduce the need for costly repairs down the road. For converters that have failed despite preventive maintenance, understanding your replacement options will limit frustration and get your vehicle back up and running cleanly again.

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