What is a Catalytic Converter?

A catalytic converter (or 'cat' for short) is a part of a car's exhaust system that helps to reduce harmful emissions. In this article, we look at how the cat works and how to protect it so you can be sure your vehicle remains road-legal.

Car exhaust

What does a catalytic converter do?

Our motor vehicles emit a number of harmful gasses from the exhaust, including carbon monoxide, hydrocarbons and nitrogen oxides. The catalytic converter sits within the exhaust system to convert these gasses into less harmful ones. This unsung hero, a legal requirement since 1993, does a very important job of protecting our environment and ensuring your car will meet the strict emissions standards needed to pass an MOT.

How do catalytic converters work?

A catalyst is something that instigates a chemical reaction, and in a catalytic converter, the substances that create this reaction are precious metals such as platinum, palladium and rhodium. These are coated onto honeycomb-like blocks inside the cat’s metal casing. The heat from the engine and exhaust starts a chemical reaction that converts harmful gasses into carbon dioxide and water vapour.

Do diesel cars have a catalytic converter?

Well, yes and no. Because diesel engines work differently to petrol, they require something called a Diesel Particulate Filter (DPF). These work in a similar way to catalytic converters, removing soot and other dangerous particles. All diesels from 2009 onwards needed to have these fitted and they’re a requirement to pass an MOT. The Diesel Particulate Filter is usually accompanied by a Diesel Oxidation Catalyst (DOC) and these have a very similar purpose to catalytic converters.

Do I need to worry about my catalytic converter?

Generally speaking, a catalytic converter is unlikely to cause you any problems as they are built to last. It only becomes a possible concern in high-mileage vehicles or those that have been used excessively for short, start-stop journeys. If you’re currently shopping for a used car, a good tip is to ask about how the car has been driven. If you want peace of mind on the condition of your cat (or diesel equivalents), most good garages will be able to check this for you and resolve any issues. You can also buy fuel additives if you’d rather try to clean the catalytic converter yourself.

Theft of catalytic converters

The precious metals used in catalytic converters have significant value and, while rare, theft of these valuable boxes has risen in recent years. Audacious thieves use jacks and cutting tools to remove the cat in a matter of minutes. The cost of replacing the catalytic converter and repairing any damage can cost hundreds or even thousands of pounds. Hybrid vehicles are a popular target as their converters can fetch a higher price. Thankfully, this kind of theft is uncommon, but there are precautions you can take if you’re worried.

How to protect against catalytic converter theft

There are a few things you can do to protect yourself against this kind of theft:

  • Park your vehicle in a secure, well-lit location, ideally off-road or in a locked garage
  • Consider installing an inexpensive security camera
  • Ask a garage to weld the converter bolts to make removal difficult
  • Fit one of the widely-available cat locks
  • Have a serial number etched onto your catalytic converter

Spotting a problem with your catalytic converter

If your cat has become damaged, blocked, or been stolen, there are some tell-tale signs that attention is needed. Keep in mind though that these symptoms can also be indicators of other problems, so the converter may not be the issue. The best thing to do is take the car to a local garage to get it checked out.

Signs of an issue with the catalytic converter:

  • The engine management light comes on
  • The engine feels less responsive
  • A smell of rotten eggs (sulphur) around the exhaust
  • Dark smoke from the back of the car

For the vast majority of us, the catalytic converter is a silent guardian protecting our environment. When problems occur, it can become expensive, so it pays to know the facts and take precautions.


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