Heating and Air Conditioning Flashcards

An engine reaches normal temperature, but heater output is poor and both heater hoses feel hot. What is the most likely problem?

Answer: Blend door not moving to the heat position

Hot coolant is reaching and leaving the core, so the problem is airflow direction, typically the blend door.

The heater inlet hose is hot but the outlet hose is much cooler, and heat output is weak. What is indicated?

Answer: Restricted or plugged heater core

A big temperature drop across the core means coolant is not flowing freely through it.

A vehicle takes a very long time to produce heat and the temperature gauge stays below normal on the highway. What is the most likely cause?

Answer: Thermostat stuck open

A stuck-open thermostat lets coolant circulate through the radiator constantly, so the engine runs cool.

A sweet smell in the cabin and a greasy film on the inside of the windshield most likely point to what?

Answer: Leaking heater core

Coolant from a leaking heater core produces a sweet odor and fogs the glass with a film.

After a cooling system repair, the heater blows cold at idle but warms up at higher RPM. What is a likely cause?

Answer: Air trapped in the cooling system

Air pockets keep coolant from reaching the heater core, especially at low pump speed.

Where does the refrigerant absorb heat from the passenger compartment air?

Answer: Evaporator

In the evaporator, low-pressure liquid refrigerant boils into vapor and absorbs heat from cabin air.

Why does selecting defrost mode on many vehicles also engage the A/C compressor?

Answer: To dehumidify air and clear fog from the glass

The cold evaporator removes moisture from the air, which clears fog faster.