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A guide to your car’s exhaust system and where to find a replacement

Car exhaust system

A car exhaust system generally consists of five or six distinct sections: a manifold, downpipe, catalytic converter, diesel or petrol particulate filter, centre section and back box.

Because most of a car's exhaust is slung beneath the body, it's exposed to all that the British weather - and roads – can throw at it. This means you have to expect certain parts to wear out over time, especially if you cover a lot of miles. While minor damage may not cause your car to fail its MoT test, missing parts and exhaust mounts, or an obvious hole in the system is likely to result in a fail, so it's important to take a good look under the car from time to time to examine the condition of your exhaust.

Every Haynes Manual takes you through the process of fitting replacement exhaust systems, while a quick Google will give you a good idea of car exhaust prices from retailers such as Euro Car Parts and Mister Auto.

Here's what those sections look like and where to find them:

An Exhaust Manifold

Where is the exhaust manifold?

The manifold is also known as the header, and shouldn't be confused with the intake manifold. It's an arrangement of metal pipes that bolt to the exhaust side of the engine, and there is commonly one pipe per cylinder.

Exhaust manifolds are frequently made of either cast iron or steel. Between the manifold and the engine is a manifold gasket which maintains a gas-tight seal between the components.

There are several different manifold designs but the purpose is always the same: to channel the spent exhaust gases into a pipe (or pipes) that run down the length of the car.


Exhaust downpipe

Most cars have a downpipe - it connects the manifold to the catalytic converter (also known as the cat). On some cars the cat is integral to the downpipe, and on others there are 'pre cats', which as the name suggests are catalytic converters that aid the function of the main cat.

The downpipe commonly (but not always) features a 'flexi' section (this is most common on a front-wheel-drive transverse engine set-up). This articulated section of pipe allows the engine to move without risking the vibration damaging the manifold.

Catalytic Converter

Catalytic converter

We all know that exhaust gases are a pollutant and contain some pretty unpleasant substances. The catalytic converter's task is to convert toxic gases into less toxic gases.

The cat features a honeycomb-like structure which increases its surface area. The catalysts within the converter break up nitrogen oxides into nitrogen and oxygen (which are harmless once separated).

They then convert carbon monoxide into carbon dioxide - and finally they convert unburnt hydrocarbons into carbon dioxide and water. The result is emissions that, while not totally harmless, are less harmful than the gases entering the cat.

There will also be what's known as a lambda sensor - also known as an oxygen sensor - mounted to the pipe that enters the cat (and on many cars a similar sensor where the pipe exits the cat). This monitors the gases entering (and exiting) the cat to ensure that it's functioning correctly. If it isn't, an engine warning light will flash up on the dashboard. An OBD scanner will help you to work out if the O2 sensor(s) is to blame for the warning light.


Diesel/petrol particulate filter

In addition to the cat, all diesel cars – and the latest petrol cars – now have a particulate filter, called a DPF (diesel particulate filter) or OPF/GPF (otto partikelfilter/gasoline particulate filter). The purpose of the particulate filter is to filter soot and other harmful particles from the exhaust gas. It traps the particulates in a series of mesh/honeycomb screens, and it does so until the filter becomes 'full'.

Once full the car starts a process known as regeneration (often shortened to regen). This is where the ECU raises the exhaust gas temperatures to such a point that the particulates burns off and are converted into less harmful ash.

Often this process is carried out as you drive without you even realising. On diesel cars, you may notice the engine being slightly more hesitant, there may even be a faint burning smell, and you'll notice the cooling fans stay on for longer (and occasionally remain on once you’ve stopped the car, indicating that the regen hasn’t completed its full 'cycle'.

Also on diesel cars, if you don't complete successive regens, and particularly when the car has only been used for short journeys, the DPF can become blocked. A 'forced regen' is then required whereby the ECU is triggered via a diagnostic tool into undertaking a regen. If this fails a replacement DPF is required.

Petrol engines with an OPF/GPF don't suffer from the same clogging issues.

Centre Section

Exhaust centre section

It's not uncommon to find the exhaust centre section and back box made as one piece, but because exhausts tend not to wear out evenly along their length, you can buy separate sections and cut and join in new sections.

The centre section of the exhaust frequently contains at least one silencer – this is a large 'box' which contains an arrangement of pipes (and occasionally wadding) and this functions to quieten the noise generated by the engine.

The more silencers you have the quieter the exhaust note, but too many silencers can restrict the flow of gas, hampering performance and efficiency. 

Back Back

Tailpipe and back box

At the rear of the car where the exhaust exits there will commonly be a back box silencer with a tailpipe, or tailpipes connected to it. This is often connected to the centre section via a sleeved fitting. The design and size of the tailpipes can alter the sound produced by the exhaust system. 

Exhaust hanger

Exhaust hangers

As you drive the exhaust moves (which is why engine mounts allow for movement). If the exhaust was bolted directly to the underside of the car the movement of the engine could easily crack, and damage the exhaust.

To prevent this exhaust systems are suspended via a series of rubber (or polyurethane) hangers. These can often fail before the exhaust, and will cause your car to fail its annual MoT test, so its worth checking yours periodically.