Is Your Cat Destroying Your Engine? The Hidden Threat of Cat Delete

What is a catalytic converter?

A catalytic converter is a device that is part of a vehicle’s exhaust system. Its main purpose is to reduce toxic gas emissions from the engine by converting pollutants into less harmful compounds. Inside the catalytic converter is a honeycomb-like structure coated with precious metals such as platinum, palladium and rhodium. These metals serve as catalysts to initiate chemical reactions that change harmful emissions like carbon monoxide, hydrocarbons and nitrogen oxides into safer gases like carbon dioxide, nitrogen and water vapor.

Catalytic converters are typically located between the engine and the muffler as part of the exhaust pipe. This allows the converter to treat the exhaust as soon as it leaves the engine. Modern catalytic converters have been standard equipment in gasoline-powered vehicles in the United States since 1975 when emissions regulations were enacted to help combat air pollution.

Overall, the catalytic converter plays a crucial role in reducing the toxic gas emissions that come from internal combustion engines. Through chemical reactions facilitated by precious metal catalysts, it converts these harmful pollutants into less toxic substances before the exhaust leaves the vehicle.


Why do catalytic converters go bad?

There are several common reasons that catalytic converters fail over time:

Age and mileage – Catalytic converters have a finite lifespan and will gradually lose efficiency as they accumulate mileage, eventually falling below emissions requirements. According to Walker Exhaust, most OEM catalytic converters are designed to last 50,000-100,000 miles.

Oil consumption – Internal engine oil consumption can foul the catalyst and coat the honeycomb structure inside the converter. This renders the converter ineffective at processing exhaust gasses. Pistons rings or valve seals that are worn out can allow oil to be burned in the combustion chambers and end up in the exhaust.

Use of leaded fuel – Lead contaminants from leaded fuels will disable the catalyst washcoat and prevent the chemical reactions inside the converter. Now banned in road vehicles, leaded gasoline was a major historical cause of converter failures.

Engine misfires – Misfiring cylinders can dump unburned fuel into the exhaust, causing overheating and damage to the converter. This may result from issues like bad spark plugs, faulty ignition components, or incorrect fuel mixture.

Symptoms of a failing catalytic converter

A failing catalytic converter will present some key symptoms that alert drivers of the issue. Three main symptoms of a faulty catalytic converter are a decrease in fuel economy, the smell of rotten eggs from the exhaust, and diminished engine performance.

As the catalytic converter starts to fail, it becomes less efficient at converting the engine’s toxic exhaust emissions into less harmful gases. This results in a reduction of fuel economy, as more fuel is required for the chemical reactions inside the converter. Drivers may notice a significant drop in their gas mileage as the converter deteriorates.

The chemical reactions inside a faulty catalytic converter also produce a pungent rotten egg smell. This is from hydrogen sulfide gas escaping through the exhaust. The sulfur odor from the exhaust is a telltale sign that the catalytic converter needs replacement.

Finally, the engine itself will start to feel sluggish and have diminished performance as the catalytic converter gets clogged with carbon deposits. Acceleration becomes slower and the engine may misfire or stall. The restricted exhaust flow from a failing converter negatively impacts how the engine runs.

Consequences of driving without a catalytic converter

Driving without a functioning catalytic converter can have several negative consequences:

Increased air pollution – Catalytic converters help reduce harmful emissions from a vehicle’s exhaust. Without one, levels of hydrocarbons, carbon monoxide and nitrogen oxides will increase significantly, contributing to air pollution and smog (source).

Potential failing of emissions test – Most areas require vehicles to pass an emissions test periodically. A missing or non-functioning catalytic converter is almost certain to cause a vehicle to fail this test (source).

Reduced engine efficiency – The catalytic converter helps optimize the fuel/air mixture for efficient combustion. Without it, engine performance and fuel economy will suffer (source).

Why people remove catalytic converters

Some people choose to remove their vehicle’s catalytic converter, despite the risks. Two common reasons are to sell the precious metals inside or to gain a slight boost in horsepower. According to Loper’s Performance Center, the metals inside a catalytic converter such as platinum, palladium and rhodium can be valuable to thieves and scrap metal dealers. The metals in the catalytic converter can be extracted and sold for profit.

Removing the catalytic converter can also lead to a marginal increase in horsepower, though usually quite small. Without the catalytic converter, there is less restriction in the exhaust system so the engine can expel exhaust gases easier. However, the gains are often less than 10 horsepower. Some people remove catalytic converters to avoid the cost of replacing a failed unit. But theCONS of removal often outweigh the pros.

Legality of removing catalytic converter

Federal law in the United States prohibits tampering with or removing catalytic converters under the Clean Air Act. Specifically, Section 203(a)(3) of the Act prohibits the removal or rendering inoperative of any air pollution control device or element of design installed on or in a motor vehicle in compliance with regulations under the Act.

Removing a catalytic converter is illegal and subject to fines of up to $2,500 for tampering per offense. Repeat offenders may be fined up to $5,000 per offense. State vehicle inspections also check for catalytic converters, so removal would cause a vehicle to fail inspection.

“Removing a catalytic converter is illegal under federal law, so assuming OP is in the states it’s illegal.”[1]

“You are free to do the same yourself However it is illegal to operate your motor vehicle with the converter either removed or non-functioning.”[2]

How to test if catalytic converter is bad

There are a few methods you can use to test if your catalytic converter is failing:

Check for OBD-II error codes – The onboard diagnostics system in your car will detect issues with the catalytic converter. Error codes P0420, P0430, P0421 and P0431 indicate problems with the catalytic converter (Source).

Visual inspection – Look under the car for any exterior damage, leaks or holes in the catalytic converter. Also inspect the piping leading to and from the converter for corrosion. Any of these signs can indicate it needs replacement.

Use temperature probes – By measuring the temperature of the exhaust before and after the catalytic converter with thermocouple probes, you can determine if the converter is working properly. A failing converter will not show a temperature increase after gases pass through it (Source).

Repairing vs. replacing catalytic converter

When your catalytic converter fails, you have two main options: repairing or replacing it. There are pros and cons to each approach.

Repairing a catalytic converter involves welding and patching up the existing unit. According to JunkCarMedics, repairs typically cost $150-300 whereas a replacement can run $1000 or more. Repairs may restore functionality in the short-term but the part is likely still old and worn, so durability and longevity are concerns. Repairs may carry only a short warranty compared to a brand new OEM catalytic converter.

Replacing the catalytic converter with a new OEM or aftermarket part ensures you get a fully functional, durable unit covered by a solid manufacturer warranty. However, the costs are much higher, often $800-2500 according to CarBrain. Replacement also takes more labor time. For an older, high-mileage vehicle, replacement may not be cost-effective.

In summary, repairing is the more affordable short-term option to pass emissions testing, but replacing is better for reliability and longevity. Evaluate the repair versus replace decision based on factors like vehicle age, expected remaining lifespan, and annual driving mileage.

Preventing catalytic converter failure

Regular maintenance and avoiding certain driving habits can help prevent catalytic converter failure. According to this article, following the manufacturer’s recommended maintenance schedule is key. This includes regular oil changes, engine tune-ups, and inspection of engine controls and electronics that affect the catalytic converter. Frequent oil changes help remove deposits and prevent oil consumption problems that can contaminate and damage the catalytic converter.

It’s also important to address any oil consumption issues, such as worn piston rings or valve seals. Consuming too much oil can foul spark plugs and coat the catalytic converter, reducing its efficiency. Make repairs for oil leaks or blue exhaust smoke, which indicates oil burning.

Only use fuel recommended for your vehicle. Poor quality gasoline with contaminants can ruin catalytic converters over time. Some additives and fuel system cleaners that claim to boost performance contain chemicals that deposit residues on converter surfaces. Use these products sparingly and avoid ones not approved for your vehicle.


In summary, the catalytic converter is a critical emissions control device in your vehicle’s exhaust system. It works to reduce toxic gas emissions by triggering a chemical reaction to convert pollutants into less harmful compounds. When the catalytic converter becomes damaged or clogged, it can no longer properly perform this function, leading to decreased engine performance, poor fuel economy, and failure of emissions testing.

The most common causes of catalytic converter failure include engine misfires, oil contamination, use of leaded fuel, and installation of aftermarket modifications like a high-flow exhaust system. Warning signs include loud rattling noises, smells of rotten eggs, increased exhaust smoke, and an inability to pass smog tests.

Replacing a failed catalytic converter is recommended over removal, as driving without one is illegal and causes significant air pollution. Converters can be tested with tools that measure exhaust gases, and should be repaired or replaced if they are no longer operating at peak efficiency. To prevent premature converter failure, drivers should keep up with engine maintenance, avoid oil contamination, use the proper fuel, and refrain from modifying the exhaust system.

In conclusion, the catalytic converter plays a crucial role in reducing harmful emissions from your vehicle. Allowing it to fail can lead to legal issues, poor performance, and increased air pollution. Drivers should be vigilant for signs of a malfunctioning converter and take prompt action to restore proper emissions control.

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