MOT advisory explained
“Exhaust mounting defective”
One of the rubber hangers or steel brackets that hold the exhaust up under the car has broken, rusted or gone missing. The exhaust is still held by the others, so it passes. On a handful of French small cars this line appears on one test in every ten to twelve.
An exhaust hangs from the underside on a series of rubber loops slipped over steel hooks. The tester found one that has perished, snapped, rusted through or dropped off. The pipe is still secure enough not to fail, but it is being carried by fewer mountings than it was designed for.
5.5% fail on the exhaust mounting at the next test and 23.3% are advised again. Left alone, the remaining hangers take the extra weight and vibration, and the pipe starts to knock on the underbody. The next thing to go is usually a weld on the silencer.
A rubber hanger costs pounds and takes minutes with the car on a ramp. A rusted steel bracket that needs welding costs more. If the silencer has cracked from hanging wrong, the row below applies.
Fix the hanger before the silencer. “back box” and “broken” are among the most common notes on this advisory. A back box that has been swinging on a broken mount cracks at its bracket, and then the exhaust replacement bill is ten times the hanger.
An “exhaust mounting defective” advisory means one of the rubber hangers or brackets holding the exhaust under the car has broken, perished or gone missing, but the system is still secure, so the car passes. A replacement rubber hanger costs £10–£40 fitted; a rear silencer that has cracked as a result costs £90–£220. According to DVSA MOT data analysed by PriceMyRepair, 5.5% of 1.5 million such advisories followed to the next test failed on the exhaust mounting.
What holds an exhaust up
Rubber on steel. The exhaust gets hot, moves with the engine and has to be isolated from the body, so it hangs on rubber. Each hanger is a thick loop or block, with a hook welded to the pipe on one side and to the body on the other. There are usually four to six along the length of the system, and the ones at the back carry the heaviest part, the rear silencer.
How they fail. The rubber perishes and stretches, then tears. The steel hooks rust through where the rubber traps moisture against them. “corroded” is the most common note, followed by “broken”, “bracket broken” and “missing”. A missing hanger is often one that stretched until it dropped off.
Why the back end goes first. Nearly four in ten of these advisories are at the rear. The rear silencer is the heaviest single part of the system, it sits behind the axle in the spray, and on many small cars it hangs on just two rubbers.
Will it fail next year?
Three wordings are grouped here. Together they have been recorded 3,205,734 times, and 1,459,563 can be followed to the same vehicle’s next MOT.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 5.5% |
| Same advisory recorded again | 23.3% |
According to DVSA MOT data analysed by PriceMyRepair: 1,459,563 advisories followed to the same vehicle’s next test. Rows can overlap.
It sits below the exhaust advisory it tends to lead to. An exhaust with a minor leak fails 9.0% of the time. A mounting is a warning, and a leak is the result of ignoring it: the pipe moves, a joint or weld opens, and the next tester hears it.
Nearly a quarter are advised again. The car is not noisy yet and the fix needs a ramp, so it waits for the next service. Mostly that is fine, and sometimes the silencer does not last that long.
| Advisory wording | Fails on same part | Advised again | Followed |
|---|---|---|---|
| Exhaust emits excessive smoke or vapour | 9.4% | 14.2% | 153,116 |
| Exhaust has a minor leak of exhaust gases | 9.0% | 26.5% | 3,311,485 |
| Direction indicator slightly discoloured | 6.1% | 19.6% | 1,901,107 |
| Exhaust mounting defective | 5.5% | 23.3% | 1,459,563 |
| Engine mounting damaged | 2.2% | 13.6% | 101,134 |
| Exhaust noisy | 0.9% | 39.7% | 382,681 |
| Undertray not fully secure | 0.6% | 8.6% | 83,373 |
Rubber ages and steel rusts, so the advisory climbs from 1.7 per 1,000 tests at 3–4 years to 15.1 in the late teens. The share that fail next time barely moves, from 4.0% to 6.2%. A broken hanger is the same problem at any age.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 3–4 years | 1.7 | 4.0% | 55,225 |
| 5–6 years | 3.7 | 4.7% | 116,250 |
| 7–9 years | 6.4 | 5.3% | 263,994 |
| 10–12 years | 11.3 | 5.6% | 366,969 |
| 13–15 years | 16.1 | 5.8% | 347,537 |
| 16–19 years | 17.9 | 5.8% | 224,030 |
| 20 years and over | 15.1 | 6.2% | 70,293 |
A knock or clonk from under the car when you pull away, change gear or go over a speed hump is an exhaust touching something it should not. So is a rattle at idle that stops when you rev. Both mean a hanger has gone.
With the engine off, reach under the back of the car and try to move the tailpipe up and down. It should move an inch or so on its rubbers and spring back. If it drops or swings freely, or if you can see a rubber loop hanging empty, that is the advisory.
A silencer resting on the axle or a driveshaft is beyond the advisory stage. It can wear through a brake pipe or a CV boot. Have it lifted the same week.
Exact wording on your certificate
Mountings and silencers each have a wording for slightly loose. The failure is an exhaust that is not secure at all.
| Line on the certificate | Result | Code |
|---|---|---|
| Exhaust mounting defective | Advisory — pass | 6.1.2 (a) |
| Exhaust silencer slightly loose | Advisory — pass | 8.1.1 (b) (i) |
| Exhaust slightly loose | Advisory — pass | 6.1.2 (a) |
| Exhaust system insecure | Major — fail | 6.1.2 (a) |
Rear leads, then front and centre. “centre” and “x2” in the notes show that testers often find more than one gone at once, which is what happens when the remaining hangers have been carrying the extra load.
| Position written by the tester | Share of all instances |
|---|---|
| Rear | 38% |
| No position recorded | 22% |
| Front | 18% |
| Central | 11% |
| Offside rear | 3% |
| Central front | 3% |
From 3,205,734 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “corroded” (20,431), “back box” (17,918), “broken” (17,462), “bracket broken” (11,744), “missing” (10,556), “centre” (8,240), “x2” (7,925), “silencer” (7,526).
What it costs
| Job | Price fitted | Notes |
|---|---|---|
| Rubber exhaust hanger, fitted | £10–£40 | The usual repair. Minutes on a ramp |
| Rear silencer, if it has cracked | £90–£220 | The cost of leaving the hanger |
| Centre section, if it has cracked | £140–£320 | Less common |
Independent garage prices, parts and labour. A rusted steel bracket that needs cutting off and welding costs more than a rubber and less than a silencer. Exhaust prices by model are on the exhaust replacement cost page.
Can I still drive with this advisory?
Get the hanger replaced within a few weeks, sooner if you can hear the exhaust knocking. Stop and have it looked at if it starts to drag or scrape on the road.
Enter your reg to see the full MOT record. A mounting advisory followed by a leak advisory a year later is one story, and the hanger was the cheap chapter.
Which cars get this advisory most?
The all-vehicle rate is 8.2 per 1,000 tests. On the models below it is between four and twelve times that.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Citroen DS3 | 99 | 12.1× | 864,893 |
| Peugeot 208 | 73 | 8.9× | 1,829,846 |
| Peugeot 207 | 71 | 8.7× | 2,075,563 |
| Citroen C3 | 64 | 7.8× | 2,369,957 |
| Peugeot 2008 | 60 | 7.3× | 1,020,413 |
| DS DS3 | 60 | 7.3× | 297,746 |
| Vauxhall Crossland | 42 | 5.1× | 343,986 |
| Vauxhall Viva | 41 | 5.0× | 310,214 |
| Volvo V70 | 35 | 4.2× | 542,743 |
| Suzuki Ignis | 29 | 3.6× | 243,488 |
Model families are grouped by the first word of the DVSA model name, so “Transit” covers every Transit variant. Only models with at least 60,000 tests are ranked.
Six of the top ten come from one company. The Citroen DS3 gets this advisory 12.1 times as often as the average vehicle, the Peugeot 208 and Peugeot 207 around nine times, and the Citroen C3, Peugeot 2008 and the later DS DS3 follow. The Vauxhall Crossland and Vauxhall Viva share their platforms. Whatever the design reason, the rear hangers on these cars do not last, and every garage knows it.
If yours is one of them, keep a spare rubber in the glovebox. It is the cheapest part on the car and the one most likely to be needed.
What to do next
- Find which hanger. The certificate gives the position. A torch under the car will show an empty hook or a hanging loop.
- Replace the rubber, or the bracket. A rubber is a five-minute job. A rusted hook needs a new bracket welded or clamped on.
- Check the silencer where it hangs. Look for a crack around the welded bracket. If it is there, price the silencer now rather than after it drops.
- Replace the other rubbers if they look tired. They are the same age. A full set costs less than one return visit.
Exhaust mounting advisory FAQ
What does exhaust mounting defective mean on an MOT?
One of the rubber hangers or steel brackets that hold the exhaust up has broken, perished, rusted or gone missing. The exhaust is still secure, so it is an advisory and the car passes.
Will a broken exhaust hanger fail the next MOT?
Sometimes, and it often leads to something worse. According to DVSA MOT data analysed by PriceMyRepair, 5.5% of 1.5 million such advisories failed on the exhaust mounting at the next test. An exhaust with a leak, which a broken hanger tends to cause, fails 9.0% of the time.
How much does it cost to replace an exhaust hanger?
A rubber hanger costs £10–£40 fitted at an independent garage. If the rear silencer has cracked from hanging on a broken mount, a replacement costs £90–£220. These are estimates and final quotes come from garages.
Is it safe to drive with a broken exhaust mounting?
Yes, for a while. The remaining mountings hold the system. Have it fixed within a few weeks, and straight away if the exhaust is knocking against the underbody or dragging.
What does a broken exhaust hanger sound like?
A knock or clonk under the car when pulling away, changing gear or going over bumps, or a rattle at idle that stops when you rev. The exhaust is touching the body or a suspension part.
Which cars get this advisory most?
Peugeot and Citroen models. The Citroen DS3 gets it 12.1 times as often as the average vehicle, and the Peugeot 208, Peugeot 207 and Citroen C3 are all close behind.
How these figures were calculated. According to DVSA MOT data analysed by PriceMyRepair, from the DVSA anonymised MOT test results (Open Government Licence v3.0): 350,943,111 tests. This advisory was recorded 3,205,734 times; 1,459,563 of those were followed to the same vehicle’s next test between 180 and 730 days later. “Fails on the same part” means a failure at that next test under the same defect code and naming the same component, on either side of the vehicle. Testers do not always name a component the same way twice, so the figure is a floor. Method and how to cite.