The difference comes down to one extra job. A two-way catalytic converter cleans up two pollutants, carbon monoxide and unburnt hydrocarbons, by oxidising them into carbon dioxide and water. A three-way catalytic converter does the same and also reduces nitrogen oxides to nitrogen, which is why it replaced the two-way design on petrol cars. The rest of this guide explains how each type works, how to tell which one you have, and what to do when it fails.
People who search “whats a catalytic converter” or “catalytic converter def” usually want one clear sentence, so here it is. If you want to know what is a catalytic converter in the simplest terms, it is a metal canister in the exhaust that uses a coated honeycomb to turn harmful exhaust gases into less harmful ones before they leave the tailpipe.
The definition catalytic converter makers and regulators use is a functional one: it is emission control equipment. The GOV.UK MOT inspection manual lists catalytic converters alongside oxygen sensors and exhaust gas recirculation valves as emission control equipment on petrol vehicles. So what’s the purpose of a catalytic converter? It keeps a vehicle within the emission limits it was approved to meet.
So what is in a catalytic converter? A stainless steel shell, a ceramic or metallic honeycomb inside it, and a thin coating on that honeycomb that carries small amounts of platinum, palladium and rhodium. Two-way and three-way units share this basic build. The difference lies in the metal mix and in how the engine controls the exhaust gas going in. Blancomet’s guide to the main converter designs explains how ceramic and metallic substrates differ.
A two-way converter only oxidises, while a three-way converter oxidises and reduces at the same time. Put simply, what catalytic converters do is speed up chemical reactions in hot exhaust gas. In a two-way unit, carbon monoxide and hydrocarbons react with oxygen to form carbon dioxide and water. A three-way unit adds a reduction step that strips oxygen from nitrogen oxides and leaves nitrogen.
The early converters were oxidation devices. According to the Science History Institute, converters were soon adapted to deal with three types of gases – nitrogen oxides, hydrocarbons and carbon monoxide – and today’s three-way devices can convert as much as 98% of them into less harmful gases.
| Feature | Two-way converter | Three-way converter |
|---|---|---|
| Gases treated | Carbon monoxide and hydrocarbons | Carbon monoxide, hydrocarbons and nitrogen oxides |
| Reactions | Oxidation only | Oxidation and reduction |
| Engine control needed | Works with surplus oxygen in the exhaust | Needs the fuel mixture held close to the ideal ratio, managed by oxygen sensors |
| Where you find it today | The same principle is used in diesel oxidation catalysts | Petrol cars |
Source: Science History Institute; GOV.UK MOT inspection manual; Blancomet.
A three way catalytic converter runs oxidation and reduction side by side, and that only works in a narrow window. The engine’s oxygen sensors keep the air-fuel mixture close to the chemically ideal ratio. With too much oxygen, the nitrogen oxide reduction falls away; with too little, carbon monoxide and hydrocarbons are not fully oxidised. Three-way units usually carry rhodium alongside platinum or palladium to help with the reduction step, which is one reason they are valuable to recycle.
The 3 way catalytic converter took over because emission rules came to target nitrogen oxides as well as carbon monoxide and hydrocarbons, and a two-way unit cannot reduce them. The two-way principle did not disappear, though. Diesel exhaust carries a lot of surplus oxygen, so diesels use an oxidation catalyst for carbon monoxide and hydrocarbons and deal with nitrogen oxides separately. The GOV.UK MOT manual lists diesel oxidation catalysts, diesel particulate filters and selective catalytic reduction among the emission control equipment on diesel vehicles.
Exhaust gas passes through these parts in the order shown. The badge shows whether each part typically carries platinum, palladium or rhodium.
Source: GOV.UK MOT inspection manual; J. Jansson, Vanadia-Based Catalysts for Mobile SCR (Springer, 2014); Blancomet.
The type of your vehicle catalytic converter follows the fuel. A modern petrol car has a three-way unit, with oxygen sensors screwed into the exhaust before and after it. A diesel has an oxidation catalyst, usually followed by a particulate filter. The code stamped on the shell identifies the exact unit, and Blancomet’s overview of two-way and three-way designs covers the variants in more detail.
If the unit is already off the car, look for the code before anything else. It is usually stamped into the shell or onto a small plate, sometimes under surface rust. Brush it gently until it is readable and photograph it straight on, together with the whole unit and both ends of the honeycomb. That record identifies the type and the vehicle application far more reliably than size or shape.
If you need a catalytic converter for Vauxhall Corsa models, petrol versions use a three-way unit and diesel versions use an oxidation catalyst, with a particulate filter on newer engines. A replacement must match the engine and the emissions standard the car was approved to, so give the garage or parts supplier your registration and engine details rather than choosing by model name alone.
Common catalytic converter malfunction symptoms include an engine warning light, sluggish acceleration, a rattle from underneath the car, a smell of rotten eggs and a failed emissions test. A catalytic converter fault reported by the car’s diagnostics usually means the oxygen sensors show the unit is no longer cleaning the exhaust effectively. The cause is not always the converter itself: misfires or burning oil upstream can damage it, so a garage should find the root problem before fitting a new one.
To confirm the diagnosis, a garage will usually read the fault codes, compare the live readings from the oxygen sensors before and after the converter, and look for physical damage such as a dented shell or a loose, broken honeycomb. On a three-way unit, those sensor readings matter most, because the converter’s ability to store and release oxygen is what the car’s diagnostics monitor.
You should not remove the catalytic converter from a car that was built with one. The GOV.UK MOT inspection manual treats manufacturer-fitted emission control equipment that is missing, obviously modified or obviously defective as a major defect, which means the car fails its MOT.
Why are catalytic converters stolen? Because the platinum, palladium and rhodium inside them are valuable, and the unit sits underneath the car where it can be cut out quickly. Three-way converters are a particular target because of their precious metal content. A stolen unit is easy to spot: the exhaust becomes very loud as soon as the engine starts.
Simple habits reduce the risk: park in a garage or a well-lit area where you can, park close to walls or kerbs so the underside is harder to reach, and report anyone jacking up a car in a car park or on a residential street.
How much for a catalytic converter depends on which side of the deal you are on. Buying and fitting a replacement is a parts-and-labour cost, while selling an old unit is a question of how much precious metal it contains.
How much to install catalytic converter parts depends on the vehicle, whether you choose an original or an approved aftermarket unit, and how much labour the job takes, including seized fixings and sensors. There is no single fixed figure, so ask for a written quote that lists the part and the labour separately. When you decide where to get catalytic converter work done, choose a garage that confirms the replacement is suitable for your car and its MOT.
If you are wondering how to put on a catalytic converter yourself, it is usually a job for a garage with a ramp. The mechanic disconnects the oxygen sensors, unbolts the old unit, fits the new one with new gaskets and fixings, reconnects the sensors, clears any fault codes and checks for exhaust leaks. Never work under a car that is supported only by a jack.
Sellers who search “how much is catalytic converters worth” usually hope for a single figure, but scrap value depends on the precious metal content of the specific unit rather than on whether it is two-way or three-way. Three-way units often contain rhodium, yet loadings vary widely by vehicle and age. Blancomet values each unit by in-house laboratory analysis of its real metal content and pays by bank transfer, in line with the Scrap Metal Dealers Act 2013.
To sell an old unit, keep it whole, photograph the code and bring photo ID, plus the V5C logbook if the converter came off your own car. Single units can be dropped at a Blancomet branch or sent by insured courier, and free UK-wide collection is available for 10 or more units.
Two-way converters oxidise two pollutants; three-way converters add a reduction step for nitrogen oxides and depend on precise engine control to do it. Petrol cars today use three-way units, while diesels apply the two-way principle in their oxidation catalysts alongside other equipment. Knowing which type you have helps you understand an MOT result, spot a fault early and judge what an old unit is worth.
If you have an old or replaced converter, do not leave it in the garage. Contact Blancomet’s recycling service for a laboratory-based valuation and payment by bank transfer.
What is the difference between a two-way and a three-way catalytic converter?
A two-way catalytic converter oxidises carbon monoxide and unburnt hydrocarbons into carbon dioxide and water. A three-way catalytic converter does the same and also reduces nitrogen oxides to nitrogen, which is why modern petrol cars use the three-way design.
Does my car have a two-way or three-way catalytic converter?
A modern petrol car has a three-way converter, which you can usually recognise by the oxygen sensors before and after it. A diesel has an oxidation catalyst that works on the two-way principle, usually followed by a diesel particulate filter.
Why does a three-way converter need oxygen sensors?
A three-way converter only works well when the fuel mixture is close to the ideal ratio. The oxygen sensors measure the exhaust and help the engine adjust the mixture, so the converter can both oxidise and reduce pollutants at the same time.
Do diesel cars use three-way catalytic converters?
No. Diesel exhaust contains a lot of surplus oxygen, so diesels use an oxidation catalyst for carbon monoxide and hydrocarbons, a particulate filter for soot, and often selective catalytic reduction for nitrogen oxides.
Will my car pass its MOT without a catalytic converter?
No, not if the car was built with one. The MOT inspection manual treats manufacturer-fitted emission control equipment that is missing, obviously modified or obviously defective as a major defect, which is a fail.
What are the signs of a failing catalytic converter?
Typical signs are an engine warning light, sluggish acceleration, a rattle from underneath the car, a smell of rotten eggs and a failed emissions test. A garage should check for misfires or oil burning upstream before replacing the unit.
Is a three-way catalytic converter worth more as scrap?
Not automatically. Scrap value depends on the precious metal content of the specific unit, and loadings vary widely by vehicle and age. Blancomet values each unit by laboratory analysis of its real metal content.
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