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".
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.
Noisy
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.