Catalytic Converter Common Faults

The Catalytic Converter on your vehicle is designed to help reduce the amount of harmful emissions produced being released into the atmosphere. If a catalytic converter should become faulty there are a number of issues that may affect the vehicle, these can be either minor or considerable problems that can affect the performance drastically. We will list a couple of common symptoms of a faulty catalyst.

Exhaust Rattle

Sometimes you may get a rattle coming from around the catalyst this can be a sign that the ceramic monolith has broken down inside the case of the catalytic converter, you may also experience some of the other issues outlined in this article.

Please Note: If your catalytic converter has only just been fitted to the vehicle and is rattling, this is a sign that the cat has not been correctly run-in according to the fitment directions.

Producing Smoke

Your vehicle may also start producing black or dark smoke from the tail pipe. Although this can be a number of different issues one problem could be a faulty catalytic converter, another common issues is carbon build up but with modern fuels and additives this has become less likely, you can always run a fuel additive through the vehicle to see if this reduces the amount of smoke.

Impact Damage

Catalytic Converters are a relatively simple design compared with some modern parts fitted to vehicles but is still susceptible to impact damage, you can visually inspect the catalysts to look for damage it may also be accompanied by Exhausts Rattle.

Melted Monolith

Catalytic Converters are designed to run hot, but they do have their limits, if your vehicle is pushing too much heat to the catalyst is may fail due to the monolith melting under the heat. This can again be a number of issues, but common faults are Ignition Coils / Spark Plugs failing and allowing too much unburnt fuel into the exhaust system.

Please Note: You will need to have the vehicle assessed and repaired before fitting a new catalytic converter else you risk damaging the replacement unit.

Clogged Monolith

Power loss is an indication of a clogged monolith also a hike in fuel consumption, generally this is caused by a contaminant such as oil getting into the catalytic converter and blocking the pores of the monolith. There is no way to really check for this problem without removing the catalyst from the vehicle and either visually inspecting it if possible or running the vehicle to check for any differences.

Failed MOT on Emissions

If a catalytic converter causes the vehicle to fail an MOT due to high emissions it does not always mean it’s a fault with the catalytic converter its self. A faulty Oxygen Sensor, for example, if causing the engine to run to lean the metals in the catalyst will be unable to react with the hydrocarbons causing the vehicle to have a higher than normal emission rate.


If you are looking to replace your Catalytic Converter but it has failed because of one of the above reasons please make sure you have the problem rectified before fitting the new Catalyst as you may also cause damage to the new item and this would not be covered by the standard warranty.

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