Catalytic converters explained: how they work and are thefts still an issue?

If you ask anyone on the street what the function of a catalytic converter is, you’ll probably be met with blank stares. While the technology is by no means new, it’s something that ultimately works in the background to reduce car emissions and usually only referenced in two scenarios: when it breaks or, something that became common over the last decade, if someone tries to steal it.

In this guide we explain everything you need to know about catalytic converters - from how they work to the materials and precious metals used in them. We’ll also cover the topic of catalytic converter theft – is it still an issue and how to protect your car from it regardless…

How do catalytic converters work?

In a nutshell, catalytic converters work by transforming the harmful substances emitted from a car’s exhaust, such as carbon monoxide, nitric oxide, nitrogen dioxide and hydrocarbons, into less harmful ones like carbon dioxide and water vapour – all by means of chemical reactions.

The interior of the ‘cat’ is usually filled with a honeycomb structure onto which a coating is applied that contains a catalyst – the substance that creates a reaction with the exhaust gases, changing their chemical structure. 

Precious metals like palladium, rhodium and platinum are commonly used as the catalyst and these have an intrinsic value that means they’re worth salvaging and recycling when the car is scrapped.

Catalytic converters are required to function at high temperatures of up to 400 degrees to maximise their efficiency. To achieve this optimum operating temperature the first units were positioned close to the car’s engine, but this caused its own issues and over time catalytic converters have gradually migrated further down the exhaust system in order to escape the intense heat of the engine.

Nowadays, catalytic converters are typically positioned towards the exhaust outlet. This easy-to-access location, plus the aforementioned precious materials found inside, makes them prime targets for thieves – more on that later.

Of course, catalytic converters only exist on internal combustion-engined cars; given there’s no exhaust gases to speak of when it comes to EVs, there’s no need for this type of technology to be fitted.

Types of catalytic converters

There are various types of catalytic converter. A simple ‘two-way’ oxidation cat works to turn carbon monoxide (CO) to carbon dioxide (CO2) and hydrocarbons, which are basically particles of unburnt fuel, to carbon dioxide and water.  More advanced ‘three-way’ catalytic converters are fitted to modern cars and these do the above while also reducing emissions of nitric oxide (NO) and nitrogen dioxide (NO2) which together are more commonly known as NOx, a major cause of localised air pollution.

Diesel cars typically have special catalytic converters to cope with the specific emissions from compression ignition diesel engines. These Diesel Oxidation Catalyst units are routinely paired with additional exhaust treatment technologies such as exhaust gas recirculation, Diesel Particulate Filters to trap soot and Selective Catalytic Reduction which uses injections of AdBlue urea solution to remove NOx.

Catalytic converter theft – is it still a problem?

Catalytic converter theft saw a huge spike towards the end of the 2010s, as well as another in the early 2020s. However, a recent Auto Express investigation found that reports of catalytic converter thefts have plummeted in recent years, going from just over 7,000 in 2021 to just 150 in 2024.

So why is this? Well, wholesale prices of rhodium, for example, have fallen from a 10-year high in 2021, meaning there is less demand for the precious metals found in catalytic converters. 

Police forces across the UK have also ramped up preventative measures including creating a specific database to track catalytic converter thefts, as well as by implementing forensic technology to catch perpetrators. That said, while the number of incidents has certainly reduced, conviction rates remain incredibly low.

How to protect your car from catalytic converter theft

Regardless of how common or uncommon catalytic converter theft is, the cost associated with replacing the stolen part can be as high as £2,000-£3,000 – primarily due to the huge damage inflicted by thieves when they commit the crime.

With this in mind, here are some handy tips to bear in mind:

• Park your car in a locked garage or a well lit area in public view with the rear of the car close to a wall or fence.
• Get your car’s VIN number etched into the casing of your catalytic converter. 
• Ask a local garage to weld the bolts on your catalytic converter or use other commercial anti-theft devices that will make it harder to remove. 

History of the catalytic converter 

Catalytic converters have been around since the 19th century when metal cylinders containing filters coated in platinum, Iridium and palladium were fitted to early French motor cars in an attempt to clean up the smoke coming out of their exhausts. The technology was first patented by Frenchman Eugene Houdry who relocated to Los Angeles in the 1930s and founded a company called Oxy-Catalyst, which fitted catalytic converters to industrial chimneys to combat smog. 

Having already fitted his filters to warehouse forklift trucks, by the 1950s Houdry had begun to research catalytic converter technology for use on cars and he secured a patent for his design in 1956. The use of the technology on production cars didn’t become widespread until lead, which blocks the chemical reaction that takes place within catalytic converters, was removed from petrol and the hands of the manufacturers were forced by tightening car emissions regulations. 

Today, the overwhelming majority of internal combustion engined cars on the road have a catalytic converter and there are various different kinds fitted to different models. Many cars also have supplementary systems such as exhaust gas recirculation, diesel particulate filters and AdBlue-based selective catalytic reduction technology that work with the ‘cat’ to help clean car exhaust emissions further. 

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