Things to do when you get home/How your car works/Catalytic Converters

The Catalytic Converter (commonly known as a Cat) is a device fitted in the exhaust system of your car that effectively reduces the harmful exhaust gases entering the atmosphere.

But how do you know if your car is fitted with one and can you notice any difference?

Catalytic Converters have been fitted to cars since 1994 as a measure to reduce the pollution in the air. The Cat works quietly and, with no moving parts, you would not be aware of it being there. If your car is fitted with a Cat then it has to run on unleaded petrol only. Otherwise, there really isn't any noticeable difference in performance or economy.

How does it actually work?

The Cat works by filtering the exhaust gases through an internal honeycomb section known as a Monolith or a Catalyst. This Monolith (made of either Ceramic or Metal) is coated with precious metals such as Platinum, Palladium and Rhodium that act as a filter for the harmful gases. Once these gases flow through the Monolith they are largely converted in a chemical reaction into non-harmful emissions such as water and carbon dioxide. The Catalytic Converter will work fine under normal driving conditions but it is important to observe certain precautions if it is to continue working efficiently.

Carbon Monoxide (CO) converts to Carbon Dioxide (CO₂)

Hydrocarbons (HC) converts to Water (H₂O)

Oxides of Nitrogen (NOx) converts to Nitrogen (N₂)

The Cat that converts these three gases is known as a 'three way Catalytic Converter'.

The exhaust gases are constantly monitored by a Lambda Sensor that is positioned in the exhaust downpipe before the Cat. This sensor samples the gases and reports back to the Fuel Injection System. If there is not enough Oxygen in the gases then the system leans off the fuel injection very slightly.

This is carried out many times per second and allows the engine to burn the fuel more efficiently, lowering the exhaust emissions.

If the exhaust emission levels are greater than normal it can be difficult to diagnose whether it is the Cat, Lambda Sensor or some other part of the engine that is at fault. However, if you are at all concerned that your car's exhaust emissions are particularly dirty or smoky, do take it along for a quick check to your Unipart Car Care Centre

How long will my Cat live for?

Generally, the Cat will need replacing once in the lifetime of the car. Nowadays though, the Cats are made from more durable materials so the lifetime will increase. The Lambda Sensor may need replacing a couple of times over the same period. However, read the precautions listed below and you should have a trouble-free Cat.

Caring for you Cat

There are some precautions to be aware of if you don't want to 'kill the Cat'. Be careful not to hit the Cat. If the exhaust system is grounded or impacted in any way this can cause the ceramic monolith inside to collapse leading to failure of the unit

Be careful not to poison the Cat This can occur in many ways but the usual is to put leaded (4 star) petrol in the fuel tank causing the Catalyst to be 'poisoned' by the lead. Other chemicals such as silicone that can get burnt with the fuel can also 'poison' the Cat.

Be careful not to overheat the Cat This can occur if the engine is misfiring. What happens is that any unburnt fuel from the misfiring engine then ignites in the Cat causing it to overheat. One to watch for is parking your car on dry grass in the summer months as Cats get very hot in normal operation and it is not unknown for fires to start as a result of this.

Have the exhaust emission levels checked regularly If it is a new car then it won't be due for an MOT test for three years. There is however a possibility that the emission levels could alter in this time allowing it to go unnoticed and resulting in the Cat becoming contaminated, the engine burning too much fuel and creating excessive pollution. If in doubt, then take it to your local garage to have it checked out.

Do not immediately blame the Cat A replacement Cat is expensive and it may not be the Cat's fault if the exhaust emissions are high. Take your car to a garage to have it diagnosed. This is done using the latest electronic diagnostic equipment and requires specialist knowledge to determine if it does need replacing.

Do not ignore a blowing exhaust An exhaust blow (holed silencer, leaking gasket or a cracked pipe) can lead to early and unnecessary replacement of the Cat. At the first sign of a noisy exhaust take your car to a garage to have it checked and repaired. It could save you a lot of money.

Have your car serviced regularly This prevents any faults going undetected and allows your garage to check that the fuel and ignition systems are functioning correctly. This will save you money by catching any problems before expensive replacement of components becomes necessary.