Catalytic converter guide: Warning lights, problems, cleaners and prices

Catalytic converter guide: Warning lights, problems, cleaners and prices

Has a dashboard warning light come on in your car? Or perhaps the annual MOT didn’t go well, with poor exhaust emissions causing your car to fail its test?

If so, don’t panic when the words ‘catalytic converter’ are mentioned. It could be cleaned, saving you a small fortune in repair bills. Or it can be replaced using a choice of affordable aftermarket parts or original equipment items.

Catalytic converters have been mandatory pieces of equipment on any new petrol-engined car built from 1993. They are fitted to help minimise a car’s impact on the environment and prevent smog.

Unfortunately, when petrol is burned by an engine to create energy, a cocktail of toxic fumes are created. These nasties include hydrocarbons, carbon monoxide and nitrogen oxides.  Diesel cars too are fitted with catalytic converters or cats and they perform a similar function. It’s the job of the catalytic converter to reduce the amount of this harmful stuff released into the atmosphere. It does this by transforming the gases into oxygen and water vapour.

Dashboard lights: the warning signs that a catalytic converter is faulty

Thousands of catalytic converters reach the end of their life every month. In September alone, more than 40,000 cars and vans failed their MOT due to dangerously high emissions, often due to a failed cat. There are various reasons a catalytic converter may be playing up. But what drivers should know is the warning light their car will display, when all is not well down below.

The warning symbol might be the engine management warning light. But in some cars, a specific catalytic converter warning light will come on.

Engine warning light and catalytic converter warning light

Car makers tend to use generic graphics for these, such as those pictured above. But it’s sensible to check in your car’s handbook, to ensure you’re familiar with all warning lights.

If either of these are illuminated in the display that houses the speedometer and engine speed, you should have a reputable independent garage or franchised dealer check the car. It could be a false alarm, or it may be more serious.

What to do if your car fails its MOT due to emissions

If a petrol car’s emissions are too dirty, it will fail its MOT. It’s likely that the authorised MOT tester will be able to offer you sound advice on whether or not they believe the fault lies with the catalytic converter, and what steps you could take to fix the problem.

It’s important to establish what’s causing the fault. If the cat is working but other parts of the car’s engine are faulty, extra-dirty fumes may be pumped into the catalytic converter, leaving it overwhelmed.

A garage will charge you to investigate the problem. But given it’s a relatively common fault, especially in petrol-powered cars over 10-years old, they should be adept at identifying the telltale signs of what’s wrong.

Before this, however, a more affordable approach is to try a product specifically designed to clean catalytic converters. There is a wide range available, from places such as Halfords or Eurocarparts. And for less than £15 you can quickly find out whether your car’s cat simply needed a clean.

Cars originally built with a catalytic converter must have them to pass their MOT

If a driver removes a petrol or diesel car’s catalytic converter that was fitted when it was built, it will fail its MOT.

The price difference of aftermarket vs original equipment catalytic converters

If investigations prove that your catalyst has come to the end of the road, you have a choice: buy aftermarket replacement parts, or original parts from your car’s manufacturer.

Most drivers will have a personal preference already. But anyone who prefers original parts may be tempted to change their minds when they see the price of reputable, aftermarket catalytic converters.

A catalytic converter made by React and supplied by Eurocarparts currently costs £200 for a Mini Cooper 1.6 petrol, and Halfords charges £300, both excluding fitment. A genuine part from a Mini dealer would cost £938, before fitting. It’s food for thought.

How does a catalytic converter work?

You can watch this video to get a real insight into the inner workings of a catalytic converter. But put simply, harmful emissions from the engine are passed through the catalytic converter. It is crammed with precious metals, including platinum, rhodium and palladium, and when the nasty emissions come into contact with the precious metals at a sufficiently high temperature, they produce a chemical reaction which cleans up the compounds.


James Mills

James is a motoring journalist and former magazine editor at BBC Top Gear and Auto Express. He has scooped, reported on and reviewed most new cars of the past 20 years, and currently contributes to the Driving section of The Sunday Times.