Is it an MOT failure to have an air horn instead of a standard one?

It's just a continuous. steady sound so functionally it's the same as the original.

It should not affect the MoT as long as it works properly.

However, it counts as a 'modification' so your insurers must be notified, or your insurance may be voided.

No matter how trivial, swapping to a different type of part (or even adding different trim etc) technically makes the vehicle non-standard and the insurers can void the policy - or increase it by whatever they feel like...

Hello

Yes it is, You must have a normal one for the MOT, However there is nothing to stop you fitting the Air horn and a change over switch. They will not fail it if you have one fitted, Only if you do not have a working normal one.

Andy C

The rules are that it must be a steady note and not strident, the last part is a subjective assesment by the examiner.

Why not wire it so you can have both at a flick of a switch

As long as it doesn't play Colonel Bogey or Dixie &c. it should be fine

http://www.motuk.co.uk/manual_160.htm

It will pass with that on with no problems.