The Ultimate Guide to Car Exhausts

Ellie Dyer-Brown, 16 days ago

5 min read

  • Exhaust
  • How it works
View of a car's exhaust pipe

WhoCanFixMyCar explains how car exhaust systems work.

Car exhaust systems direct harmful gases away from the cabin, keeping you and your passengers safe. This guide explains how they work, why they sometimes fail and how they can be repaired.


How does a car exhaust work?

Exhaust components

How much does it cost to fit a new exhaust?

How long does it take to fit a new exhaust?

Does exhaust repair putty work?

How to repair a scratched chrome exhaust

How to repair a hole in the exhaust pipe

How does a car exhaust work?

A car's exhaust has four main functions:

All exhausts create six gases, three of which are toxic: nitrogen oxide, carbon monoxide and nitrogen monoxide. The exhaust system channels these harmful gases away. The process looks like this:

  • The exhaust manifold collects exhaust gases from the cylinder head.

  • The manifold then releases these gases through a single opening (often called the front pipe).

  • From there, the gases move through the catalytic converter, where harmful elements are removed.

  • The next step sees them passing through the silencer, where their noise is reduced. Some cars - such as those geared towards high performance - opt for a back box that alters the noise rather than lowering it.

  • Finally, the exhaust gases are expelled through the tailpipe.

Exhaust components

A car's exhaust is typically composed of six sections: a downpipe, manifold, catalytic converter, particulate filter, back box and centre section.

Exhaust downpipe

The downpipe connects the exhaust manifold to the catalytic converter. Most cars have one. It usually features a flexi section that allows the engine to move without damaging the manifold.


The exhaust manifold, sometimes called the header, shouldn't be confused with the intake manifold. It's an arrangement of pipes bolting the exhaust to the engine's side - usually one pipe per cylinder.

Exhaust manifold white background

A manifold gasket sits between the manifold and the engine, maintaining a tight seal to prevent gases from escaping. Manifolds are often made from cast iron or steel, and although they can have different designs, their purpose is always the same: to move exhaust gases into a pipe or pipes under the car.

Catalytic converter

A catalytic converter on a modern car.

The catalytic converter changes toxic exhaust gases into less harmful ones. A lambda or oxygen sensor is mounted to the pipe that enters the cat (and often the one that exits it) to monitor the gases, ensuring it works correctly.

Learn more about catalytic converters in this guide.

Particulate filter

All diesel cars have a diesel particulate filter (DPF) to catch and store soot, further reducing the toxicity of exhaust gases. The DPF does this until it is almost full. Then, under the right conditions, it regenerates (self-cleans).

However, regeneration doesn't always happen, leading to blockages. Find out how to avoid DPF problems.

Modern petrol cars usually have a petrol particulate filter, but these don't experience the same blockage issues.

Back box

An exhaust back box/silencer

You can usually find a back box silencer and one or more tailpipes at the rear of the exhaust system. The back box is connected to the centre section using a sleeved fitting.

Centre section

The exhaust back box and centre section are sometimes made as one component, but you can buy them separately because exhaust parts wear out at different rates. The centre section often contains at least one silencer - sometimes more.

Signs of exhaust failure

If there’s an issue with your exhaust - such as a hole in the system - the chances are, you’ll know about it. Here are the symptoms to look for. 

1. Unusual noises

  • Hissing and rumbling from the rear of your car indicate you might have an exhaust leak.

  • The same applies if your exhaust sounds louder than normal.

  • Rattling could be caused by a loose or misaligned exhaust.

  • Chugging might be the result of a blockage in the system.

2. Poor fuel efficiency

If your exhaust is leaking or there is a blockage somewhere, your engine will have to work harder, resulting in poor fuel economy.

This guide discusses how to get the best fuel efficiency from your car.

3. Decrease in power

A faulty exhaust can lead to an incorrect air-fuel ratio, reducing your car's power and performance.

4. Strong exhaust smell

An exhaust smell that's stronger than usual often points to a problem with the exhaust system. You should never ignore this symptom because you could be breathing in toxic gases.

How much does it cost to fit a new exhaust?

On average, having an exhaust fitted costs £125.06 when you book through WhoCanFixMyCar. It’s worth mentioning that some exhaust problems can be repaired without replacing the whole system, which will usually work out cheaper.

Mechanic doing exhaust repair

How long does it take to fit a new exhaust?

The most common part of the exhaust system that is replaced is the back box, which can take between thirty minutes and an hour. Other exhaust components take a lot longer to replace - for the whole system, your car could be in the garage for a day.

How to repair a scratched chrome exhaust

Chrome plating is very thin, so scratches and scrapes will almost always develop. Thankfully, fine scratches can be removed by following these instructions.

  • Find a polish designed for chrome and apply it to a soft cloth.

  • Using circular motions, rub the cloth over the affected area. Apply more polish as necessary and continue rubbing for several minutes.

  • Next, use a microfibre cloth to remove the polish and inspect the area. If you can still see the scratches, repeat this process until they are gone. However, be aware that some deeper scratches will require re-plating to get rid of them.

How to repair a hole in the exhaust pipe

If your exhaust pipe has a small hole, you might be able to repair it yourself using repair tape or exhaust putty. Tape provides an almost instant fix, whereas putty takes much longer to dry and lasts longer.

To locate the hole, you’ll first need to jack your car up so you can see the source of the problem. Make sure the area around the hole is clean.

Disclaimer: The following instructions give a rough idea of how to use exhaust tape and putty. You should always follow the instructions that come with the product.

Using Tape

  • Let the engine run for a few minutes to warm it up if it's under 20 degrees outside. The tape will stick more easily to warm metal.

  • Next, turn off the engine (don’t let it get too hot, or you could burn yourself) and remove the backing from the tape. It’s a good idea to wear gloves while you apply the tape to prevent any burns.

  • Wrap a few pieces of wire around the tape to secure it properly.

  • Finally, let the engine run for twenty minutes to fuse the tape to the exhaust.

Using putty

Repair putty - sometimes called exhaust paste - is designed to provide gas-tight seals for joints and holes in the exhaust system.

  • After you’ve cleaned the area, dampen it with a cloth.

  • Spread the putty around the hole, ensuring it’s filled up and covered.

  • Let the engine idle for around ten minutes.

  • Once applied, you should avoid driving your car for 24 hours to give the putty time to dry.

Does exhaust repair putty work?

It can be used to seal up small holes and leaks, but any significant damage or corrosion will need more serious attention from a professional.

It’s important to remember that the above methods are temporary fixes; you should still get your exhaust repaired by a professional as soon as you can.

Frequently asked questions

How do you make an exhaust louder?

Exhausts can be made louder by fitting a slightly wider tailpipe or replacing the silencer with a performance back box. However, there are laws around exhaust noise that must be followed.

Specifically, it is illegal to modify the exhaust system to make it louder after it has been 'type approved' (checked that it meets safety and environmental standards).

Why do MOT tests include an exhaust emission test?

An emissions test is conducted during your annual MOT to ensure your car operates efficiently and doesn't produce excessive pollution, which harms the environment. This guide gives a complete overview of what's checked in an MOT.

What is diesel exhaust fluid?

Diesel exhaust fluid (DEF) is used to reduce the amount of air pollution produced by diesel cars. A common brand name for DEF is AdBlue. You can learn more about it here.

What are resonators on an exhaust?

Resonators are designed to reduce humming, buzzing and other high-pitched noises from the exhaust. Not all cars have resonators; in those that do, they are located between the silencer and catalytic converter. Sounds at a particular frequency bounce off the inside of the device, cancelling each other.

Can you drive without an exhaust?

There's nothing physically stopping you from using your car without an exhaust. However, you would put yourself at risk from harmful fumes, and your car would fail its MOT, after which it would be illegal to drive.

Do electric cars have an exhaust?

Electric vehicles don't require an exhaust system because they run on batteries, so they don't produce emissions, unlike conventional internal combustion engine cars.

Find out if electric cars are easy to maintain.

Looking for an affordable exhaust repair? You're in the right place - WhoCanFixMyCar can help you find the right garage at the right price.

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