Code permits furnaces and air handlers to be installed in a garage. Duct work must be sealed/or chased.
Well I've turned down jobs that required the ductwork to enter and leave the garage because I believed that it was too much of a risk.
How does the code you mention avoid the obvious danger?
I may be retired but I am always ready to learn something new.
Duct work running through a garage is chased/enclosed in drywall. So no vapors can enter it.A furnace or air handler is allowed to have its plenum and a starting duct not enclosed if it goes directly through the ceiling.