BS 5306-3 covers the maintenance of portable fire extinguishers, including their components, which would include the horn & elbow and horn & hose of CO2 extinguishers.
The user monthly check would usually identify any obvious damage and the basic service each year would involve a thorough inspection for wear & tear, damage and functionality, including renewal of O-rings &/or fibre washers to ensure a good seal of the hose/elbow to the valve.
Any obvious damage would result in replacement - air entrainment through a cracked horn can affect extinguishing capability and a totally broken off horn and elbow presents an injury damage risk as with the resulting loss of pressure reduction the extinguisher once discharged can literally fly off like a rocket and has caused serious injury and fatalities. A worn hose could rupture affecting efficacy as well as being a hazard.
During the 10 year overhaul it is normal (but not mandatory)to renew the horn & hose/horn & elbow assemblies.
(A revised edition of this standard is due for release imminently)
BS EN 3 details the functional requirements for hoses and horns fitted to CO2 extinguishers
