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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.
Catalytic Converters have been
fitted to almost all petrol cars after 1994 in the UK. Cats are also now fitted to many diesel
cars.
Cats are fitted to reduce the pollution in the atmosphere. The Cat works
quietly and, with no moving parts, you would not be aware of it being there.
If your car is running on petrol with a Cat fitted, then it must be run on unleaded petrol only.
There is a very slight loss in engine power output with a catalytic converter fitted.
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 Cat that converts these three gases below and this is known as a 'three way Catalytic Converter'.
Carbon Monoxide (CO) converts to Carbon Dioxide (CO2)Hydrocarbons (HC) converts to Water (H2O)Oxides of Nitrogen (NOx) converts to Nitrogen (N2)
The exhaust gases are constantly
monitored by a Lambda Sensor that is generally positioned in the exhaust
down-pipe before the Cat. This lambda sensor samples the gases and reports back
to the engine management 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. Diagnosis is generally only done by using electronic test equipment.
Generally a Cat does not require servicing and will last for very high mileages and or many years.
The Lambda Sensor may need replacing from time to time as they can fail much more frequently than the cat itself.
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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. Driving through water when the cat is at working temperature which suddenly cools the cat can cause the cat to break up internally.
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. Any un-burnt fuel from a misfiring engine collects in the Cat and then ignites in the Cat causing it to overheat. it has been known for the cat to get so hot under these conditions to set fire to the under body and interior of the car.
Another 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.
Replacement Cat's are expensive and it may not be the Cat's fault if the exhaust emissions are high. Take your car to a qualified garage to have it diagnosed.
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.
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