Naked Science Forum

Non Life Sciences => Technology => Topic started by: acsinuk on 29/12/2020 15:32:40

Title: Why do cars fail an MOT just because an indicating warning light is on?
Post by: acsinuk on 29/12/2020 15:32:40
Why do cars fail MOT just because an indicating warning light is on.     For example my Nissan Pixo failed because the airbag light remained on ?
Look at this clause 7.1.6[a[ in the attached document     https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/696292/mot-inspection-manual-for-classes-3-4-5-and-7-from-20-may-2018-draft.pdf
In my case, the airbag light remained ON and I was told that the car fail as the airbag must be faulty which would cost around £400 from the main dealer.  So I investigated and removed steering wheel cover etc and found the airbag was OK and had a reading of 2 ohms.
Next, I looked on line and people were talking about loose wiring under the drivers seat which goes to the seatbelt switch which was faulty.  However, it took me some time to track down a replacement seatbelt post with contacts in side and hence will have to pay for another MOT test.
BUT, why is this classified as a major fault a failure anyway, when it is obviously a minor defect and in fact, many cars have a switch to disable the airbag from operating?
I am sure there are many other indication/electronic failures which are listed as major faults when they are minor and hopefully can be revised now we are leaving Europe. 
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Colin2B on 29/12/2020 16:08:27
MoT tests are based on potential safety hazards, if your airbag isnít working as indicated by a warning light then it will fail.
I have our car serviced at the same time so any problems are spotted before the MoT
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Bored chemist on 29/12/2020 18:19:53
BUT, why is this classified as a major fault a failure anyway
Because it means you don't know if the airbag will save your life or not.

and hopefully can be revised now we are leaving Europe. 
Being outside Europe won't save you if what you need is an airbag.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: vhfpmr on 29/12/2020 18:55:27
If you read the introduction in your reference it defines the categories, minor is reserved for faults that don't affect safety.

BTW, if you want an honest MOT, take it to the local authority testing station, not a garage. They don't do repairs, so they have no reason to fail it unless it's warranted. After I started taking my car to the council VTS I never had another failure. My garage once said "When's your MOT due? It'll need a new suspension arm to pass the test.". They looked pretty sheepish when I told them it had just passed a week ago. (If your car still keeps failing, buy a Honda.)
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Colin2B on 30/12/2020 11:16:47
and hopefully can be revised now we are leaving Europe.
Being outside Europe won't save you if what you need is an airbag.
Also, many people will want to drive in Europe without having to have an extra test and I doubt insurance companies would be too happy.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: alancalverd on 30/12/2020 12:26:31
The MoT test, like most good ideas, predated the European Union.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: acsinuk on 30/12/2020 15:15:42
What I found on the Pixo was that the airbag was working fine but the seat belt switch was mixed up electronically with the indicator lamp.  So the vehicle complied with the regs but the warning lamp was lighting up incorrect so could be a design fault as other owners have had a similar problem.
  The  MOT test is for roadworthiness not for checking if all instrument panel lights are working.  Surely, this is a minor fault and certainly not safety related at all.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Bored chemist on 30/12/2020 15:21:52
The  MOT test is for roadworthiness not for checking if all instrument panel lights are working. 
If the safety systems are not working, it is not  roadworthy.
Why do you not realise that?
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: chris on 30/12/2020 18:00:29
The MOT emissions tests are pretty crazy and, in some cases, are causing some perfectly good cars to be pulled off the road and replaced with new ones, engendering a larger carbon footprint (from the embodied carbon cost of manufacture) than had the original car been allowed to continue on its merry - slightly more polluting - way.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: alancalverd on 30/12/2020 22:27:11
The  MOT test is for roadworthiness not for checking if all instrument panel lights are working. 
In this case the panel light was working, and indicating a potentially dangerous fault. False positives are a pain, but much less dangerous than false negatives or ignored positives.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: vhfpmr on 31/12/2020 13:09:01
the airbag was working fine
How did you establish that?
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: acsinuk on 31/12/2020 15:04:39
 Correct, it is a really involved and needs an experienced specialist engineer with advanced electronic equipment to check it out.  This is not the sort of check that a normal MOT test station can undertake; which is another good reason why it should be a minor rather than a major defect.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: vhfpmr on 31/12/2020 15:53:37
Correct, it is a really involved and needs an experienced specialist engineer with advanced electronic equipment to check it out.  This is not the sort of check that a normal MOT test station can undertake; which is another good reason why it should be a minor rather than a major defect.
Read your own reference, a minor fault is one that doesn't affect safety, airbag operation is safety related, therefore it's not minor. It's not the job of the VTS to diagnose the reason for the warning light, they are there to verify compliance with the MOT requirements, not repair your car for you. You didn't answer my question, how did you verify that the airbag isn't faulty? At the very least it's faulty if for no other reason than the warning light is showing when it shouldn't.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Bored chemist on 31/12/2020 16:40:47
which is another good reason why it should be a minor rather than a major defect.
So, you think that the distinction between a major fault  and a minor one is how easy it is to diagnose.

Do you understand that some people might think that the potential outcome of the fault is more important?
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: alancalverd on 31/12/2020 18:40:12
Bald tyres and missing brake pads are very easy to diagnose and fix, so should be considered minor.

A nonfunctioning CD autochanger is very complicated and probably impossible to repair (the manufacturer no longer exists)  so the car must be scrapped immediately.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: acsinuk on 04/01/2021 17:12:34
Exactly so Alan,  Now what is required is that test stations must specify exactly what needs to be done to make the car comply. In my case it was 7.1.6[a] and a note that new airbag costs £410.  But when I removed the airbag and checked it out and found it healthy and returned to the test station they said. But the light is still on and we do not know why.
That is not good enough the test station admitted it could not diagnose what was wrong so in my view they should have given a pass with a note that the owner should seek advice on how to correct the electronics on the airbag circuit which in effect is a minor defect.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Petrochemicals on 04/01/2021 17:18:39
I am sure there are many other indication/electronic failures which are listed as major faults when they are minor and hopefully can be revised now we are leaving Europe. 

Not likely, of all of the countries in Europe Britain has some of the most stringent safety standards and the lowest road deaths, it is more likely now Britain has left Europe, Europe can lower its standards.

https://www.google.com/amp/s/www.autoexpress.co.uk/car-news/106521/uk-roads-are-the-safest-in-europe%3famp

Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: alancalverd on 04/01/2021 17:59:26
Exactly so Alan,  Now what is required is that test stations must specify exactly what needs to be done to make the car comply. In my case it was 7.1.6[a] and a note that new airbag costs £410.  But when I removed the airbag and checked it out and found it healthy and returned to the test station they said. But the light is still on and we do not know why.
That is not good enough the test station admitted it could not diagnose what was wrong so in my view they should have given a pass with a note that the owner should seek advice on how to correct the electronics on the airbag circuit which in effect is a minor defect.
On the contrary. The test station only had your word that the airbag was OK, no reason to believe that it would work on impact, and a positive  indication that something was still wrong somewhere. If a doctor found you had a high temperature and no other symptoms, would you expect him to tell you what dose of radiation or antibiotic (which one?) would cure it?   

Before every flight, we work through a checklist of must work/acceptable defect/not fitted, and if a warning light  comes up on a "must", you don't fly. I'd be perfectly happy to drive a car with airbags "not fitted" but if they are in place and not working correctly, they might deploy randomly and cause an accident, so there's no "acceptable defect" category.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: vhfpmr on 04/01/2021 18:26:10
Exactly so Alan,  Now what is required is that test stations must specify exactly what needs to be done to make the car comply. In my case it was 7.1.6[a] and a note that new airbag costs £410.  But when I removed the airbag and checked it out and found it healthy and returned to the test station they said. But the light is still on and we do not know why.
That is not good enough the test station admitted it could not diagnose what was wrong so in my view they should have given a pass with a note that the owner should seek advice on how to correct the electronics on the airbag circuit which in effect is a minor defect.
Twice I've asked you how you have established that the airbag is 'healthy', and you haven't answered. It ought to be obvious to you that the purpose of the warning light is to indicate when there's a fault, and even if it's only the warning light system itself that's faulty, it's not capable of doing the job for which it was fitted and you won't have any notification if the airbag does become faulty at some point in the future. It's the job of the VTS to find the faults, not to diagnose their cause, and it's your job to get the faults fixed when they find them. If you don't like the terms and conditions that come with running a motor vehicle, you don't have to run one.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: acsinuk on 06/03/2021 17:46:39
I found the fault was in the the clock-spring squib and re-soldered the faulty connection out.
 But the faulty lamp remained on and despite disconnecting the battery overnight it did not reset by rebooting the electronics.
This was extremely annoying and cost me £70 at the main dealer  to get the electronics reset. 
I should really have complained as this, as it is a manufacturing defect and  totally unnecessary as a reboot should reset all faults automatically.  If the fault is not cleared by the reboot then of course the red light would come back on next time you use the car.  Why Nissan use an EPROM instead of a ROM reset is a real miserable mistake.  Very unfriendly.
Title: Re: Why do cars fail an MOT just because an indicating warning light is on?
Post by: Bored chemist on 07/03/2021 11:33:50
Why Nissan use an EPROM instead of a ROM reset is a real miserable mistake.  Very unfriendly.
So... you want it to be impossibly to upgrade the firmware.

Well. that's one viewpoint.