UL tests all dry chemicals to operate at up to 120 degrees Fahrenheit. This includes the Kidde automotive fire extinguisher. The operational test requires the fire extinguisher to discharge at a specified time and powder amount. UL does not have the option of testing for operation above 120 degrees.
At temperatures above 120 degrees, the fire extinguisher will still function, but the discharge time may be a little shorter than the 9 to 10 seconds required by UL.
UL requires the fire extinguisher to be able to withstand storage at 175 degrees without rupture.
High temperatures will decrease the life of the elastomeric seals, so storage above 120 degrees will decrease the life of the fire extinguisher.