Blowing exhaust? Exhaust leak signs, MOT rules and fixes

Is a blowing exhaust an MOT failure? Usually, yes. A blowing exhaust is an exhaust leak: gas escaping through a split, a hole or a failed joint before it reaches the tailpipe, which is what makes the rumbling, hissing or popping noise. The MOT inspection manual makes “a major leak” a major defect, which is a fail. So are exhaust fumes getting into the cabin, exhaust noise well above that of a standard silencer, and any leak that could affect the emissions reading^1,2. If the fumes are a danger to the people on board, the defect is “dangerous”, and the vehicle should not be driven until it has been repaired^1,3.

The short answer

• A major leak, fumes in the cabin, excessive noise or a leak that could upset the emissions test: MOT fail^1,2.
• Fumes that endanger the people on board: dangerous. Don’t drive it^1,3.
• The law requires every exhaust system and silencer to be kept “in good and efficient working order”, MOT or no MOT⁴.
• Leaks come from failed joints, a blowing flexible section, cracks, holes and rust^5,6. A leaking joint needs a new gasket and fixings; a split or rusted pipe needs a new section.
• Fix the leak before you judge an emissions result: readings taken with a leaking exhaust might be wrong².

Below: the signs of a leak, exactly what the MOT checks, whether you can still drive, where exhausts leak, how to find the leak and how to fix it properly. These are the rules for MOT tests in England, Scotland and Wales; the test works differently in Northern Ireland³.

Signs of a blowing exhaust

The manufacturer’s guide to exhaust leaks lists these as the common signs⁵:

• Noise. A loud rumble, especially when you accelerate, and sometimes an intermittent hissing or popping while the engine runs.
• A smell of exhaust fumes inside the vehicle. Damaged pipes can let fumes into the cabin, which is a health risk to everyone on board. Take this one seriously: see can you drive with a blowing exhaust?
• Loss of power when you try to accelerate, which gets worse if the leak is left.
• Worse fuel economy. Depending on where it is, a leak can upset the engine’s air and fuel mixture, for example making it over-fuel.
• Soot. Black soot marks around a joint, crack or hole show where gas is escaping.

A leak can also bring on the engine warning light. The fault code P0420, “catalyst efficiency below threshold”, can be set by an air leak in the exhaust giving the sensors false readings, not only by a failing catalytic converter⁶. Our guide to the P0420 code goes through the checks in order.

A rattle is a different problem. Before blaming the catalytic converter, check for a vibrating heat shield or a loose baffle in the exhaust, which can be mistaken for a rattling converter⁶. See what causes a faulty catalytic converter.

Will a blowing exhaust fail the MOT?

The tester checks the exhaust system on every vehicle with an internal combustion engine, hybrids included, and judges its noise and the effect of any leak on emissions. These are the items that apply to an exhaust leak, from DVSA’s MOT inspection manual^1,2:

What the tester finds	Category	Manual section
Exhaust system has a major leak or is insecure	Major (fail)	6.1.2 (a)
Exhaust fumes entering the cabin	Major (fail)	6.1.2 (b)(i)
Exhaust fumes a danger to the health of people on board	Dangerous (fail)	6.1.2 (b)(ii)
Exhaust noise in excess of what is permitted	Major (fail)	8.1.1 (a)
Any part of the silencer system insecure, or likely to come off	Major; Dangerous if likely to come off	8.1.1 (b)
An induction or exhaust leak that could affect emissions (petrol or diesel)	Major (fail)	8.2.1.1 (b), 8.2.2.1 (b)

What counts as “major”. The manual gives no size for a major leak; the tester judges it. Noise is judged against a reference: exhaust noise “must not be unreasonably above the noise level you’d expect from a similar vehicle with a standard silencer in average condition”, listened to during the emissions test or with the engine at around 2,500 rpm². A leak that is not major can still fail the test on noise, or if it could affect the emissions reading.

The emissions test. A leak can let air in as well as gas out, and upset the engine’s air and fuel mixture, so it can spoil the reading^5,6. The manual tells testers that if the exhaust is holed badly enough to fail, they should recheck the emissions once it is repaired, and warn the presenter that readings taken with a leaking exhaust might be incorrect². So if your vehicle failed on both the exhaust and emissions, fix the leak first, then see what the emissions do. Our post on failing the MOT on emissions explains the readings.

Hybrids. Hybrids, mild hybrids included, are left out of the petrol exhaust gas test, but their exhaust system is still inspected for leaks and security like any other^1,2.

What the categories mean. Dangerous: do not drive the vehicle until it has been repaired. Major: repair it immediately. Minor: a pass, but repair it as soon as possible. Advisory: it could become more serious, so monitor it³.

The retest. The exhaust is not on the list of items that can be retested free if you take the vehicle back by the end of the next working day. Leave it at the test centre for the repair and have it retested within 10 working days and the partial retest is free; take it away and bring it back within 10 working days and you can be charged a partial retest fee. After that it needs a full MOT, at up to £54.85 for a vehicle with up to 8 passenger seats⁷.

Can you drive with a blowing exhaust?

The law. The Road Vehicles (Construction and Use) Regulations 1986 require every vehicle with an internal combustion engine to have an exhaust system with a silencer, and every exhaust system and silencer to be “maintained in good and efficient working order”⁴. That applies every day, not only on MOT day.

After a failed MOT. You can drive the vehicle away only if its current MOT is still valid and no “dangerous” problem was listed, and it must still be roadworthy. Driving a vehicle that failed its MOT with a dangerous problem can mean a fine of up to £2,500, a driving ban and 3 penalty points⁷.

Fumes in the cabin. Exhaust gas contains carbon monoxide: it is one of the gases the MOT measures at a petrol tailpipe². The NHS describes carbon monoxide as a poisonous gas that is colourless and does not smell, so you cannot tell it is there. Its symptoms include a headache, dizziness, feeling or being sick, weakness, tiredness and confusion, and shortness of breath⁸. If you can smell exhaust inside the vehicle, stop using it until the leak is found and repaired. If anyone has those symptoms, the NHS says to call 111, or 999 if they are finding it hard to breathe, have suddenly become confused or have lost consciousness⁸.

Where exhausts leak, and why

• Joints. The flanges and gaskets where one section bolts to the next, from the manifold back. When a part is replaced, the manufacturer’s fitting advice is to fit it to clean mating surfaces with a new fitting kit wherever possible, because rusty nuts and bolts can lose their threads⁶.
• The flexible section. Where the exhaust has a flexible joint (a “flexi”), a “blowing flex” is one of the structural failures the manufacturer lists, along with a broken flange and snapped pipework⁶.
• Rust. If a rusty-looking pipe compresses or flexes when you gently squeeze it with pliers, consider replacing it before it starts to leak⁵.
• Cracks and holes anywhere along the system, including the manifold itself⁵.

Why it happened matters. Worn engine mountings let too much of the engine’s movement into the exhaust, and corroded exhaust brackets that are not replaced put extra strain on it. Both can end in a snapped pipe, or a converter snapping at its weakest point⁶. Replace them with the leaking part, or the new one can go the same way.

Not a leak: the small “weep” hole at one end of many silencers. It lets condensed water escape to slow rusting, does not cause a leak and doesn’t need fixing⁵.

How to find an exhaust leak

Safety first: let the exhaust cool completely with the engine off, and once the vehicle is jacked up, put it on axle stands. Never go under a vehicle held only by a jack⁵.

1. Look. Start at the manifold and work back along the whole system, looking for cracks, holes and soot. Mark every suspect spot so it is easy to find again⁵.
2. Squeeze. Test rusty sections gently with pliers, as above⁵.
3. Use a vacuum and soapy water. If you still can’t see it, connect a vacuum cleaner hose to the tailpipe, switch it on, and spray soapy water along the system from the manifold back. Where the water bubbles, there is the leak⁵.

For garages: don’t pressurise a blocked exhaust. An exhaust is a flow-through device, not a pressure vessel. Blowing air into one that is blocked at the other end will force gas out somewhere, for example round a sound welded joint, and can condemn a part that does not leak in use. In the manufacturer’s flow test, less than 1 litre per minute at a pressure of 4 to 5 psi is acceptable⁵.

How to fix a blowing exhaust

• A leaking joint: a new gasket, with new nuts and bolts if the old ones are rusty. Our fitting kits come in three levels: A is the gaskets, B adds the fixings and C adds the springs. Each kit fits only the part it is listed for, so check the part number first.
• A split, holed or rusted pipe, or a failed flexible section: replace that section. We list front pipes and connecting pipes for many makes and models.
• A catalytic converter or DPF with a broken flange or a damaged body: replace the unit. For most vehicles first used from 1 March 2001, a business may only supply or fit a replacement catalytic converter that is type approved for that vehicle⁹; see type approval and the law.
• No exhaust paste in front of a catalytic converter or DPF. The paste sets hard, breaks off in chunks and can crack the converter’s ceramic core, and using it in front of the converter voids its warranty. When fitting a new converter or DPF, the manufacturer’s advice is to use no sealant or exhaust paste at all^6,10.
• Afterwards: start the engine and check the joints for leaks, and clear any fault codes from the engine computer^6,10. If the vehicle failed on emissions too, have them rechecked now that the leak is fixed².

Step-by-step fitting guides: fitting a catalytic converter and replacing a DPF.

Exhaust pipes for popular vehicles

How many front pipes and connecting pipes we list for each model on 28 October 2026. Each page lists every part by engine, with today’s price.

Vehicle	Front pipes	Connecting pipes
Vauxhall Astra	17	28
Volkswagen Golf	19	11
Renault Clio	9	11
Vauxhall Corsa	3	15
Ford Fiesta	6	10
Ford Mondeo	2	14
Peugeot 308	0	18
Kia Sportage	3	16
Citroen Berlingo	4	17
Peugeot Partner	4	17

Every part lists the vehicles and years it is catalogued for, so check it against your vehicle. Most of our pipes, catalytic converters and DPFs come with a fitting kit included, all carry a 2-year warranty (3 years on Premium silicon-carbide DPFs) and are backed by our fitment guarantee, and delivery is free to most of the UK, next working day on orders placed before 4pm, Monday to Thursday. Browse exhaust pipes, catalytic converters and DPFs, or choose your vehicle from the vehicle list.

Frequently asked questions

Is a blowing exhaust an MOT failure?

A major leak is a major defect, so yes. Fumes in the cabin, excessive noise and a leak that could affect the emissions reading are fails too, whatever the size of the leak^1,2.

Can a blowing exhaust cause the engine management light?

It can. An air leak in the exhaust can give the sensors false readings and set the code P0420, which looks like a failed catalytic converter⁶. Fix the leak and clear the code before replacing the converter.

Is it illegal to drive with a blowing exhaust?

The exhaust and silencer must be kept in good and efficient working order at all times⁴. After a failed MOT you may drive away only with a valid MOT and no dangerous problem listed; driving with a dangerous problem can bring a fine of up to £2,500, a ban and 3 points⁷.

Can I use exhaust paste to fix the leak?

Never in front of a catalytic converter: hardened paste breaks up, damages the converter’s core and voids its warranty. The manufacturer’s advice is to use no sealant or exhaust paste when fitting a catalytic converter or DPF^6,10. A leaking joint needs a new gasket; a holed or rusted pipe needs a new section.

Is the small hole at the end of my silencer a leak?

No. Many silencers have a small weep hole at one end to let water escape and slow rusting. It does not cause a leak⁵.

Enter your vehicle below to see the pipes, catalytic converters and DPFs catalogued for it.