In this figure, what do the orange arrows marked #3 indicate? 
Definitions:
Passenger Compartment
The section of a vehicle intended for the seating and protection of passengers.
HVAC System
An acronym for heating, ventilation, and air conditioning system, which provides heating and cooling to residential and commercial buildings.
Dirt
Loose or packed soil or earth; in a mechanical context, it can refer to unwanted particulate matter that can cause wear or contamination.
Malfunctioning Thermostat
A defective device that fails to regulate temperature properly, leading to heating or cooling systems operating erratically.
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