What are the 3 most leading failures of a catalytic converter?

A catalytic converter is a vital component of a vehicle’s exhaust system. It helps reduce harmful emissions by converting pollutants like carbon monoxide, hydrocarbons, and nitrogen oxides into less toxic substances through oxidation and reduction reactions. However, catalytic converters can fail over time due to factors like old age, poor maintenance, engine issues, and driving habits. Identifying the leading failures of catalytic converters can help drivers understand when a converter needs replacement and how to prevent premature failure. This article will examine the 3 most common catalytic converter failures.

What is a Catalytic Converter?

A catalytic converter is a stainless steel box mounted on the exhaust pipe between the engine and muffler. Inside the converter, a ceramic honeycomb structure coated with precious metal catalysts like platinum, palladium, and rhodium provides the surface area needed for chemical reactions. As hot exhaust gases pass through the converter at high speeds, the catalysts facilitate oxidation and reduction reactions to convert harmful pollutants into less toxic substances.

Specifically, carbon monoxide and hydrocarbons are oxidized into carbon dioxide and water. Meanwhile, nitrogen oxides are reduced to nitrogen and oxygen. This conversion process significantly reduces emissions of pollutants that can contribute to smog and respiratory problems in humans.

Catalytic converters are a key component of a vehicle’s emissions control system. They became standard equipment on gasoline-powered vehicles in the US after the passage of the 1970 Clean Air Act. Properly functioning converters can eliminate over 90% of pollutants from exhaust gases. Most modern vehicles have advanced three-way catalytic converters to reduce all three major pollutants simultaneously.

1. Catalyst Degradation

One of the most common failures of catalytic converters is catalyst degradation. The catalysts inside converters are gradually worn down over time through normal use. High-mileage vehicles over 100,000 miles often show significant catalyst degradation that reduces conversion efficiency.

There are several causes of catalyst degradation:

Thermal Degradation

The high temperatures and flow rates inside a converter during engine operation can cause the catalyst and washcoat materials to sinter. This reduces the active surface area available for chemical reactions. Thermal degradation occurs faster when a vehicle frequently drives at high speeds or loads.


Certain contaminants in fuel, oil, or air can attach to catalyst surfaces and block sites for chemical reactions. Lead, sulfur, phosphorus, and silica are common catalyst poisons. Using leaded gasoline fouls converters, while burning oil containing sulfur and phosphorus accelerates poisoning.

Loss of Precious Metals

The precious metal catalysts can gradually contaminate the washcoat and even vaporize during use. Rhodium is especially prone to vaporization at high temperatures. Loss of active catalyst metals reduces conversion efficiency over time.

Catalyst degradation causes a steady decline in converter performance. Most converters have excess capacity when new, so degradation may not cause immediate issues. But well-worn converters with 30-50% declines in activity can trigger engine problems and allow more pollutants through.

Keeping up with routine maintenance and avoiding catalyst poisons reduces degradation rates. But the converter will still wear out eventually as the vehicle ages. Replacement is the solution when the catalyst is no longer effective enough due to degradation.

2. Physical Damage

Physical damage is another common failure mode for catalytic converters. Impacts, cracking, plugging, and other physical damage can render the converter inoperative. Some of the causes of physical damage include:

Collision Impacts

A catalytic converter positioned low on the underside of a vehicle is vulnerable to impacts from road debris, speed bumps, steep driveways, and minor collisions. Even a mild front or rear collision can crush the converter and damage the fragile ceramic substrate inside. Collision damage typically requires replacement of the entire converter.

Vibration Fatigue

The constant vibration of normal driving can also damage converters over time. Cracks often develop in the ceramic substrate that break it into smaller, less effective pieces. Vibration fatigue occurs faster with greater exposure to rough roads or hauling heavy loads.


Severe overheating due to engine issues like misfiring, running lean, or oil burning can melt and crack the ceramic substrate. The high exhaust temperatures also accelerate catalyst degradation.


Catalytic converters can become plugged by foreign objects or debris from engine damage. Carbureted engines are prone to plugging if a backfire blows out the flame arrestor and sends debris into the exhaust. Converters can also become clogged from oil contamination or unburned fuel. Any blockage in the converter will restrict exhaust flow.

Physical damage can reduce converter efficiency through loss of active surface area and flow restriction. Replacing the damaged converter is the only remedy in most cases. Preventing impact damage and overheating helps avoid failure.

3. Theft

The theft of catalytic converters has recently surged due to the high value of precious metals contained inside. The metals like platinum, palladium, and rhodium are extremely rare and expensive. Recycling converters can recover these precious metals.

Catalytic converter theft is quick and easy with a portable saw. Thieves typically target larger vehicles that sit higher off the ground for easier access to the converter. Hybrid vehicles are also popular targets because their converters operate less frequently and have less degradation.

Preventing theft is difficult once a thief targets a vehicle. Etching the VIN on the converter may help deter some thieves or aid in recovering the stolen converter. Parking in secure, well-lit areas reduces the risk of theft. Video surveillance and tamper-proofing with welds or protective cages provide more reliable methods to thwart thieves.

Replacing a stolen catalytic converter restores emissions control until the new converter potentially gets targeted again. Using less valuable converter designs without precious metals would eliminate the incentive for theft but reduce performance. Theft prevention is ideal to avoid replacement costs and ensure emissions compliance.

Signs of Catalytic Converter Failure

Identifying when the catalytic converter is failing allows drivers to take action before additional problems develop. Here are some common signs of converter failure:

– Reduced fuel economy and acceleration
– Dark exhaust smoke, especially when accelerating
– Rotten egg odor from exhaust
– Failed emissions testing
– Glowing converter when hot
– Rattle or buzz from loose converter
– Check engine light illuminated

Diagnostic trouble codes tracked by the engine computer often point to specific converter issues. For example, codes P0420 to P0423 indicate inefficient converter operation.

When the converter fails completely, the vehicle may not run at all. Severe clogging can block exhaust flow, while disintegration of the ceramic substrate allows pieces to rattle around inside. The failed converter must be replaced to restore proper operation.

Catalytic Converter Replacement

Replacing a failed catalytic converter restores emissions control and engine performance. New OEM converters from automakers meet original specifications for fit and performance. High-quality aftermarket converters can provide comparable function at a lower cost in some applications.

Converter replacement costs between $1,000-3,000 for parts and labor at a professional shop. DIY replacement can save on labor but requires cutting and rejoining exhaust pipes. Proper safety gear is essential when working under a supported vehicle.

Replacing just the damaged ceramic substrate inside the converter is also possible in some cases through specialty rebuilders. This costs less than full converter replacement but has mixed success rates.

Preventing Catalytic Converter Failure

Taking proactive steps maximizes the lifespan of catalytic converters and avoids premature failure:

– Maintain regular oil changes, engine tune-ups, and repairs to prevent misfires and excess fuel burning.
– Avoid contaminated gas and oil with catalyst poisons like lead and sulfur.
– Protect the converter from road debris and overheating.
– Drive smoothly without sudden accelerations or engine revving.
– Ensure proper converter shielding and heat sinks are in place.
– Etch a serial number on the converter to deter theft.
– Park carefully to minimize collision and theft risks.

Modern catalytic converters can last over 100,000 miles with proper engine maintenance and driving habits. But deterioration over time is inevitable, ultimately requiring replacement to maintain emissions compliance and performance. Being aware of the most common failure causes allows drivers to maximize converter lifespan.


Catalytic converter failure is an increasingly common issue as vehicles age beyond 100,000 miles. Three primary failure modes are catalyst degradation, physical damage, and theft. Contamination, overheating, vibration, and collision impacts accelerate converter wear and damage. Precious metal theft has also emerged as a major converter failure cause.

Watching for reduced performance, exhaust odor, and warning lights identifies impending failure. Replacing the damaged converter restores proper emissions control. Preventive maintenance and operation extends the service life. But ultimately converters require replacement once the catalysts are too degraded for effective pollutant conversion. Understanding the leading failure causes helps drivers maintain this vital emissions control component.

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