[pothole]

JFP - I can assure you that the test does not involve interrogating the computer.

Blue2000S - yes, I'm pretty sure that would be the case. As I understand it, the rule for things like ABS and airbags is that if they are present, they must not be accompanied by an error light.

I will need to double check, but I believe that if the car was factory fitted with an airbag that has then been removed, you will fail if an a fault light shows, but not if it does not.

To quote from the AA:

"As well as electronic parking brake and electronic stability control warning lights (where fitted) the MOT test will also include checks for the correct function of the following, where fitted;

Headlight main beam warning light
Electronic power steering warning light
Brake fluid level warning light
Tyre pressure monitoring system warning light
Air bag warning light
Seat belt pre-tensioner warning light"

So it's all about the presence and correct function of the "warning" lights.

Also, to quote the VOSA who sets the test:

"Level one is a visual inspection
together with a simple check of
the malfunction indicator light –
or MIL – and is covered by the
introduction of new MOT test
items in line with the Testing
Directive.

For the moment, testers do not
have to worry about levels two and
three, which will cover the process
to retrieve the fault codes stored
in the Electronic Control Unit (ECU)
of the vehicle system and the
check of ECS performance by
carrying out functional tests."

So, it's all about the lights. For now.

It's a fairly silly system. But if you think about it, it will cover the vast majority of cases.