IDENTIFYING THE FAULT
This is the easiest sort of fault to identify and can be simply caused by wear and tear. Examples of this are a broken flange, snapped pipe work, blowing flex etc. When the part has snapped, check the engine mountings are not worn. This allows excessive movement through the engine into the exhaust which can contribute to this type of failure. If exhaust brackets are not replaced when they corrode, extra pressure is put on the cat and can lead to it snapping at the weakest point.
The first thing you should do is check that the cat is actually rattling. It may be a vibrating heat shield on the vehicle or a loose baffle in the exhaust causing the problem and it is being wrongly identified as a rattling cat. Once it has been established that the cat is rattling, it should be checked for the following:
Check the cat for dents and road rash (scratches). If these are evident it is likely the cat has impacted against a speed bump causing a fracture in the ceramic monolith and the cat to break down and rattle.
Check the inlet side of the port for exhaust paste. If it is evident, it is likely that the exhaust paste has broken up and impacted against the monolith causing it to fracture and break down.
Blue can/melted monolith
Does the outside of the cat have a blue/purple shade? Does the ceramic monolith show signs of melting and warping? If so, this is a sign that the catalyst has superheated due to excessive levels of unburned fuel in the exhaust system. When this fuel enters the catalyst it ignites, superheating the monolith and causing it to breakdown. This is always a sign there is a fault with the running of the vehicle so a full diagnostic check and emissions check should be carried out to identify the fault before fitting a replacement cat.
Operational failure is failure with the working of the cat, such as an MOT emissions failure or engine management light issues.
Emissions failures should always be investigated carefully as failures normally occur due to a vehicle fault. If the vehicle has failed on the hydrocarbons (>200ppm), there is a definite fault with the vehicle over-fuelling which must be rectified before fitting a replacement cat. If the vehicle has failed on the ‘λ’ (lambda), there is a problem with the air:fuel mixture on the vehicle. It does not always mean the lambda sensor needs replacing. A CO failure means the catalyst is not able to convert all the gases that pass through it. However if it has hydrocarbon levels greater than 60ppm it is likely the cat has been contaminated with unburned fuel and is unable to convert to its full capacity.
Engine Management Light (EML)
EML issues can be tricky to resolve. The most common fault code that occurs in relation to a cat failure is PO420 (cat inefficient). There can be several reasons this code can be generated which are not a fault of the cat. For example, it can related to a lambda fault, an air leak in the exhaust system causing false readings, or retarded spark timing. All possible faults should be thoroughly investigated before condemning the cat.