Frequently Asked Questions About Catalytic Converters

Choosing the right catalytic converter for your car is increasingly recognised as one of the most important decisions you will make when maintaining your car. Whether you are looking to maximise performance, minimise fuel consumption or do your bit for the environment getting the right catalytic converter for you will save time, money and avoid the headaches of failing an MoT.

Here are some frequently asked questions about catalytic converters:

Why should I have a catalytic converter?

If your car was fitted after 1st August 1992 with a catalytic converter from new, you need to have one for the MoT. It needs to be in good working order to comply with the Government’s emissions legislation. The engine has been designed to work with a Cat and besides giving out toxic emissions, the performance will be reduced if your Cat is not working properly. You will fail the annual MoT if either it is not present or it is not in good working order.

Can I remove my Cat?

If your car was registered after 1st August 1992 you must have the Cat present and working for the MoT. Previously registered cars can have the Cat permanently removed.

Why do people want to remove their Cats?

Without a Cat the engine gives more power and reduced petrol consumption. You may remove the Cat for track / competition / display use, for example.

Why might my Cat fail?

Road Damage

The outside of the cat can be damaged by hitting solid objects in the road. ie, speed bumps, large rocks etc.

Plugged or contaminated

Plugged or contaminated catalytic converters are caused by the wrong sort of fuel in your car. Using leaded or lead replacement fuel will plug up the monolith and cause it to stop working. A similar thing will happen if fuel additives are used that are not suitable for use with a catalytic converter.

Melted / Broken substrate

A monolith is usually broken when it is impacted by an object or when it suffers a sudden change in temperature. If the catalytic converter suffers road damage (See section on road damage) the monolith inside can be cracked due to it being crushed by the movement on the steel can.

The use of exhaust paste before the catalytic converter can cause the monolith to break. When the exhaust paste has hardened, small pellets may break away and shoot into the catalytic converter. The monolith will gradually be destroyed by these pellets and break down. The catalytic converter can also be damaged by excessive engine vibration. Another possible cause of a fractured monolith is a sudden temperature change. (See number 7 in “Caring for your Catalytic Converter”)

The monolith can be melted when unburned fuel is injected into the catalytic converter. This can be caused by the car being bump or tow started, or if the car’s engine requires several turns before firing. More information can be found on this is the section on “Overheating”.


There are many problems that can cause a catalytic converter to overheat or fail. The most common cause is unburned fuel entering the catalytic converter. Also faulty spark plugs and leads will cause the engine to misfire and ruin the catalytic converter. It will also be damaged if the distributor timing is out.

  • Oxygen Sensor – An oxygen sensor collects and sends information to the Electronic Control Unit. This is used to control the fuel/air mix. If the oxygen sensor is faulty, or it has been contaminated with silicone from anti-freeze or sealant, he fuel/air mixture will not be corrected and excess fuel will enter the catalytic converter, causing it to overheat.
  • Fuel injection systems – If a fuel injector is leaking internally or dribbling fuel into the engine the catalytic converter may be damaged. The excess fuel will enter the exhaust system and cause it to overheat. Due to the varying fuel injector systems the proper manual should always be consulted when trying to diagnose a problem.
  • Map Sensors – Map sensors tell the Electronic Control Unit the load on the engine and the amount of air entering it. If this sensor fails it causes a rich condition in the engine, which can overheat the catalytic converter causing it to fail.
  • Carburettor systems – A worn or defective carburettor can cause a catalytic converter to overheat. Problems such as improper float or air/fuel mix adjustments and worn metering rods can damage the catalytic converter. The choke system also needs to be operating properly so the right level of fuel enters the system. If too much fuel enters the system it will overheat and eventually fail.
  • Canister Purge Valve Control – This vacuum operated valve vents fuel vapour from the carburettor bowl to the charcoal canister. If the vacuum is breached the charcoal canister will flood, causing the air/fuel mixture to become very rich. This will cause the catalytic converter to overheat and break down.


A catalytic converter should not be considered to be a silencer although it does have some silencing qualities. Noise can be caused by excessive fuel getting into the catalytic converter.

What happens if my Cat fails?

If a catalyst fails it can block and the car will break down.

What does my Cat do?

Catalytic converters are specialist environmentally friendly devices fitted in the exhausts of vehicles which ensure that hydrocarbons are burnt off and that minimise the emission of harmful pollutants. Cars fitted with converters can use only unleaded petrol as the lead in leaded fuel poisons the converter and irreparably damages it.

Car manufacturers have ruled out the retrofitting of catalytic converters on older cars, saying that it could irreparably damage the converter, as vehicles which have been using leaded fuel for many years have lead deposits in their engines. On fitting with a catalytic converter, these deposits could poison the Cat and damage it permanently.

Unleaded fuel, when burnt, produces gases such as carbon dioxide, carbon monoxide, hydrocarbons, and oxides of nitrogen, the latter three can be poisonous or carcinogenic. A Catalytic Converter, which is working correctly, creates a chemical reaction between these gases, which converts them to relatively harmless gases. It is critical for the environment that you have your Catalytic converter checked regularly to ensure it is functioning properly.

Catalytic converters fitted to vehicles can reduce toxic emissions (e.g. Carbon Monoxide, NO2, HC) by up to 90%. Catalytic converters do not reduce CO2 emissions.

Can I fit a Cat to a car which doesn’t have one?

If your engine ran on leaded petrol you cannot. The old deposits will poison the Cat.

How can I best care for my Cat?

There are several things you can do to ensure your ‘cat’ has a long and healthy life.

  1. When fitting the catalytic converter, don’t use exhaust paste in front of the cat. When the exhaust paste hardens it can break off in chunks and damage the monolith. Paste can also block the monolith.
  2. Always use the correct fuel for your car.
  3. Never use a fuel additive without first reading the instructions to find out if it is suitable for use with a catalytic converter. If in doubt, contact the manufacturer of the additive.
  4. Never attempt to bump or tow start your car. This causes unburned fuel to be injected into the catalytic converter, which makes the monolith overheat and melt.
  5. Never tow anything that is too heavy for your vehicle to cope with. For example, an overloaded caravan will actually push a car along when it travels downhill. This sends unburned fuel into the exhaust system and can cause the monolith to melt down.
  6. Have your car regularly serviced to your car makers specifications. In particular, make sure the engine is running properly. A poorly tuned engine can cause the monolith to break down or become covered in soot, which stops it working regularly.
  7. Take care when driving through deep puddles, fords or parking when there has been heavy snowfall. The catalytic converter operates at an extremely high temperature, and when it comes into direct contact with water or snow it cools down more rapidly than the monolith, and in extreme circumstances the monolith can be crushed as the steel shell contracts.
  8. Don’t park your car over long grass or anything similar. As the catalytic converter operates at such a high temperature it can actually set the grass on fire!
  9. Drive slowly over speed bumps or very bumpy roads to reduce the chance of the exhaust system being grounded. This could cause impact damage to the catalytic converters.