Dodgy air-con on 158s has become a "go to" complaint for those with any interest in the running of the railway, and has become such a "railway trope" that you'll notice every time you see a 158 with the emergency hoppers open. Often, crews haven't given the system time to get going before they open the hoppers (due to inexperience, or to prevent being set upon by an angry mob of overdressed pax), which means if it wasn't broken before it definitely will be soon!
But yeah, let's just cancel trains because it's a bit warm. Clearly the lesser of two evils there...