Will a loud horn cause MoT failure?

Q. I am just wondering if there are any MoT or legal issues with a Steibel Nautilus horn that pumps out 139 decibels?

My existing horn is fading slightly and I lost count of just how many people pulled out in front of me today – twice very dangerously in front of cops, who did bugger all.
Philtheidiot, MCN forums

A. The MoT only requires the horn to be operational during the test. Bearing in mind the deafening volume from a Steibel, it would be a courteous gesture if you warn him before he hits the button, he may well show you some kindness with the rest of the test!