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Welcome to our latest post on what MOT data can tell us about which cars you should be looking at when buying used.
This week we’re looking at sports cars, and using MOT data to try and understand which are the most reliable sports cars.
CarXpert has developed a unique way of rating cars based on their MOT history. We give cars a rating of between one and five stars based on their MOT failures, MOT advisories, and overall MOT history. The star rating system is explained below.
A car’s MOT history can give you a good idea of both its condition and how well it’s been looked after. If you have a used car that you’re considering, enter the registration number here and CarXpert will give you its ratings. And it’s free!
How We’ve Calculated Our Rankings
To come up with our rankings we’ve analysed MOT results for cars that are between 6 and 12 years old using our CarXpert algorithms. We used this age range because we wanted to look at cars that had had a number of MOTs, as we’re assuming that people tend to keep their cars for a few years so would like to know which are the best cars over a period of time.
We’ve looked at items that have caused cars to fail their MOTs and also advisory items that owners have been made aware of by MOT testers. CarXpert’s algorithms gives each car a star rating for Failures and Advisories, and then we’ve looked at the average ratings for both for each make and model.
Some sports cars sell in very low volumes, e.g., Ferraris, Aston Martins, so while there are other models of sports cars outside of our list that get high ratings, that’s based on a very small number of cars. That makes it hard to be confident that the average ratings per model are accurate, and also makes it impossible to draw conclusions about what may go wrong with low volume models. We’ve therefore limited our list to cars that have sold in sufficient volumes to provide reliable information.
How CarXpert’s Star Ratings Work
CarXpert has analysed huge amounts of data about MOT results and can rate cars on a 1 to 5 star scale in terms of how good their MOT results are compared to cars of the same age:
- For a car to get five stars it must have had zero failure or advisories – nothing has ever been reported by an MOT tester
- A four star car is one with better than average MOT results – some issues have been found during MOTs, but fewer than the typical number
- A three star car is around the average for MOT results – this car has had about the number of issues found that would be expected
- A two star car is worse than average for MOT results – more issues have been found during MOTs than would be expected
- A car that only gets one star has much worse than average MOT results – a lot more issues have been found at MOTs than would be expected
How We’ve Identified Issues To Look Out For
As well as ranking each make and model, we’ve also used CarXpert’s technology to analyse what items typically cause each type of car to fail MOTs, and therefore what you should be looking out for if you’re thinking of buying one. The most common reasons that all types of cars fail MOTs on are normal wear and tear items like tyres, wiper blades or bulbs, but there are usually issues that occur more frequently on particular types of car, and it’s those that we’ve highlighted for each car in our list.
Below is our list of the top 5 Used Sports Cars and what to look out for when buying one.
There are a couple of things you should look out for on most of them –
If number plates have the characters modified or the spacing changed, this can lead to an MOT failure. This crops up with all of these cars, presumably because they’re more likely to have personalised plates than some other types of cars. So if you are looking at buying a car and the private registration is included, make sure the number plates are legal or you could have a problem when you come to MOT the car.
If a car has high-intensity headlamps then if the headlamp cleaning system isn’t working then that’s an MOT failure. Several of the models in our list are more likely to have Xenon or other types of high-intensity headlamps than some other types of car, so you should check the headlamp cleaners are working. If you’re not sure if the car has high-intensity headlights, the MOT inspection manual has some tips on how to check.
1 Porsche 911
In first position in our list is the Porsche 911, with an average CarXpert rating of 4.13
The chart below shows the proportion of 911s that fall into each of CarXpert’s 1 to 5 star rating categories. Amazingly, 79% of these Porsches get a 4 or 5 start rating, so if you’re looking at buying a good example of one of these then the odds are in your favour.
While as we’ve mentioned above, these cars do very well when it comes to MOT tests, there are a few things to be look out for if you’re considering a used 911.
There can be problems with driveshaft constant velocity (cv) joint boots failing. This appears to be particularly on the front of the car, so more likely to affect four wheel drive versions. This isn’t easy to check when viewing a car so have a look at the MOT history for any recent MOT advisories that relate to the cv joint boots. You can view the MOT history or any car you’re considering when you get its CarXpert rating.
Another thing to check on the MOT history is any mention of brake pipes being damaged or corroded, as this can cause 911s to fail their MOTs.
Make sure that the airbag warning light comes on with the ignition and goes out when you start and engine. If the airbag (SRS) light indicates a fault then the car will fail its MOT.
Something you can’t check is the exhaust emissions. 911s can fail MOTs due to “Lambda coefficient outside the default limits or the range specified by the manufacturer”. Once again, have a look at the MOT history – if that has come up on a car you’re interested in try and find out what action was taken to fix it.
Depending on the design on the wheels you may be able to get a good look at the condition of the brake discs. These can cause MOT failures on 911s more than is typical, so make sure they’re not heavily worn (check for a ridge at the outer edge of the disc) or corroded.
The Porsche Club of Great Britain have put together a comprehensive buyer’s guide to the 997 generation Porsche 911 that will give you a lot more useful information.
2 Porsche Boxster / Cayman
In second place we have another Porsche, or arguably two. As the Boxster and the Cayman are closely related (the Cayman is effectively a coupe version of the convertible Boxster) we’ve treated them as one model for the purposes of this list, and across the two versions they got an average CarXpert rating of 4.07.
Here’s how Boxsters and Caymans are spread across CarXpert’s 1 to 5 star categories.
Again, these cars compare well to many other types on the road, with 72% of them getting a 4 or 5 star rating, but there’s a few things to be mindful of when looking at a used Boxster or Cayman
As with the 911, brake pipe damage or corrosion can cause these cars to fail an MOT, so have a look at the MOT history for any mentions.
Brake discs can wear out, so if you can get to the discs through the wheel check there isn’t too much of a ridge at the outer edge, which would indicate the disc is getting close to wearing out. Refer to the MOT history for any recent mentions of brake disc wear.
While you’re inspecting the brake discs check that all the wheel bolts are present and correct. These can go missing, and that’s an MOT fail.
Again as with the 911, emissions can be excessive to the point where the car fails an MOT. Once again, the MOT history is the only way you’re going to be able to check if this has been a problem on a car that you’re considering, and if it has, find out what was done to fix the problem.
Make sure the ESC (Electronic Stability Control) comes on with the ignition and goes one the engine has started. Issues with the ESC crop up more than average on Boxsters and Caymans, and can cause an MOT test fail.
Pistonheads have put together a useful buying guide to the 987 generation Boxster , and also one for the same generation of Cayman.
3 Audi TT
In third place, with an average CarXpert rating of 3.81 is another German car – the Audi TT.
The graphic below shows what proportion of TTs are in our 1 to 5 star ratings categories.
If you’re looking at a used TT there are a few things to be aware of.
Some of the front suspension joint dust covers can wear out. Similarly, transmission constant velocity joint boots can wear, and in both that can lead to an MOT failure. These aren’t easy things to spot at the roadside, so have a look at the MOT history for any recent MOT advisories that mention anything in these areas.
Airbag systems can play up, and if the SRS (supplementary restraint system) light indicates a system malfunction then the car will fail its MOT. Ensure the airbag light comes on with the other indicator lights and goes out once the car has started.
We mentioned at the start of this article that headlamp cleaning needs to be checked on all of these sports cars. In the case of the TT, also check that the headlamp levelling system works, as this failing crops up more than average as an MOT failure item on these Audis.
Also check all the other lights are working – and in particular that they are the correct brightness and colour. TTs fail MOTs with comments like “Lamp showing red light to the front, white light to the rear or has heavily reduced light intensity front or rear” more than is typical.
If you’d like more information on the Audi TT, here’s a buying guide.
4 MX-5
In fourth place is the Mazda MX-5, with an average CarXpert rating of 3.73
The chart below shows what proportion of MX-5s are in each of CarXpert’s 1 to 5 star ratings categories
Things to watch out for if you’re looking to buy a used MX-5 include:
The front suspension appears to throw up a few different issues that can result in an MOT test failure. Dust covers can go missing or fail, and some other components can fracture or come loose. Unless you can get the car up on the ramp, your best bet for spotting issues like these is to see if the MOT history mentions anything.
Front brake calipers can seize, leading to the brakes being on even when the brake pedal hasn’t been pressed. On the road test, see if the car pulls to one side – that may be an indication that a caliper is sticking. Also feel the temperature of the front wheels after the test drive. If one is noticeably warmer then it’s likely the caliper on that side is sticking.
As with some of the other cars in our list, excessive exhaust emissions can cause MX-5s to fail an MOT. Checking the MOT history for any mention of issues in this area is therefore recommended, and if a car has had problems, find out what was done to rectify it.
Another thing to look out for on the MOT history is any mention of corrosion on the underbody or suspension components. Corrosion in these areas is a cause of MOT failures on MX-5s more than is typical across all models of car.
Here’s a buying guide that has more information on the different MX-5 models, engines and equipment levels.
5 Mercedes SLK
The final car in our list, with an average CarXpert rating of 3.70 is the Mercedes SLK.
Here’s the breakdown of how SLKs fare in CarXpert’s rating categories
There aren’t too many things that are specific to SLKs to be aware of when looking at a used example.
As well as the general checks on headlamp cleaning systems and registration plates that we recommend for all the cars in this list, you should make sure that all the warning lights come on when you turn on the ignition and go out again once the engine is running. Warning lights staying on for SRS (airbags), ABS, and ESC (Electronic Stability Control) all cause MOT failures on SLKs more than is usual.
SLKs can also suffer from fluctuating braking effect. On the test drive find a safe place to brake hard and check that the car pulls up in a straight line and without any vibration through the steering.
On older cars the headlamps can discolour, and that can lead to an MOT failure, so make sure the headlights are clear.
Autocar have published an informative buyers guide to the SLK if you’d like more information.