Catalytic Converter Warranty Advice

Catalytic converters fail for many reasons. In many cases, the failure has little to do with the cat itself but instead is the result of another fault within the vehicle. It is important to rectify any problems before a new cat is fitted. Below is a list of problems related to a vehicle fault and therefore not covered under a manufacturer’s warranty:


Emissions Failure

When a vehicle fails an MOT emissions test, the blame is usually placed on the cat. The cat can only convert what gases pass through it. If the vehicle is running poorly, the cat will become contaminated with unburned fuel and become unable to convert the gases efficiently.

If the cat is replaced without fixing the vehicle fault, it is highly likely it will pass the MOT test as the new cat is working at full capacity and will mask any fault. A tell-tale sign of a vehicle that has not been repaired is that they will return a year after failing the original MOT having failed the test again.

Engine Management Light/Fault Codes

The engine management light will be triggered on the dashboard if the vehicle’s emissions values fall outside the set parameters. The most common fault code relating to the cat is the ‘cat inefficient’ code: P0420. It is important to investigate the following faults before condemning the cat:

  • Lambda fault
  • Air leak in the exhaust system
  • Retarded spark timing

Impact Damage

This includes external damage caused by hitting a solid object such as a speedbump. External damage can be identified by scratches or dents to the outer can.

Melted/Broken Monolith

A monolith is usually broken up when it is impacted by an object or when it suffers a sudden change in temperature. This is identified by blue/purple colourisation of the outer can or rattling, indicating the monolith has broken up.


Although a cat does have some silencing qualities, it should not be considered to be a silencer. Noise can be caused by excessive fuel getting into the cat.


Overheating is caused mostly by unburned fuel entering the cat due to an engine misfire. Possible vehicle faults causing this are:

  • Faulty spark plugs
  • Distributor timing is out
  • Faulty lambda sensor
  • Faulty fuel injector system
  • Faulty map sensor

Plugged and Contaminated

If a cat becomes ‘clogged’ or ‘contaminated’, it will no longer be able to convert any gases. This is usually caused by one of the following reasons:

  • Incorrect fuel
  • Unsuitable fuel additives
  • Engine problems

If the cat becomes totally blocked, the engine will fail due to the increased back pressure. Poor engine performance could be an indicator of this.

Poor Fitting

Over-tightening can cause flanges to crack. Non-usage of spring bolts can cause excessive vibrations causing the monolith to break up, as can worn gaskets.

Use of Exhaust Paste

The use of paste in front of a cat is an automatic warranty failure. Exhaust paste becomes brittle when dry and can break off. If the dry paste hits the monolith, it can cause breakages and fractures.

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