ASE Heating and Cooling 3 — Questions and Answers
Question 1: The engine is at normal temperature and both heater hoses are hot, but the heater only blows cold air. What is the most likely cause?
- Restricted heater core
- Low coolant level
- Blend door stuck in the cold position (Correct answer)
- Thermostat stuck open
Correct answer: Blend door stuck in the cold position
Two hot hoses show coolant is flowing through the heater core, so the fault is in air delivery, such as a stuck blend door.
Question 2: The heater inlet hose is hot but the outlet hose is much cooler, and heat output is poor. This most likely indicates:
- A restricted or plugged heater core (Correct answer)
- A faulty blower motor resistor
- A blend door actuator failure
- An overcharged A/C system
Correct answer: A restricted or plugged heater core
A large temperature drop across the heater core means coolant flow through it is restricted.
Question 3: A blower motor works only on the high speed setting. What is the most likely cause?
- Failed blower motor
- Open blower motor resistor (Correct answer)
- Blown blower fuse
- Faulty blower ground
Correct answer: Open blower motor resistor
High speed usually bypasses the resistor, so an open resistor kills the lower speeds only.
Question 4: After a coolant flush, the heater blows cold at idle but warms up when the engine is revved. What is the most likely cause?
- Air trapped in the cooling system (Correct answer)
- Stuck-closed thermostat
- Bad radiator cap
- Faulty blend door actuator
Correct answer: Air trapped in the cooling system
An air pocket blocks low-speed coolant flow through the heater core, so the system must be bled properly.
Question 5: An engine overheats at idle and in slow traffic but stays normal at highway speed. What is the most likely cause?
- Restricted radiator core
- Inoperative electric cooling fan (Correct answer)
- Collapsed lower radiator hose
- Stuck-open thermostat
Correct answer: Inoperative electric cooling fan
At highway speed ram air cools the radiator, so overheating only at low speed points to a cooling fan problem.
Question 6: An engine stays at normal temperature at idle but overheats at sustained highway speed. Which cause is most likely?
- Inoperative cooling fan relay
- Partially restricted radiator core (Correct answer)
- Faulty ECT sensor to the gauge
- Open blower resistor
Correct answer: Partially restricted radiator core
Under high load a partly plugged radiator cannot reject enough heat, even though it keeps up at idle.
Question 7: On vehicles that have one, what does the heater control valve do?
- Regulates engine coolant temperature
- Controls coolant flow to the heater core (Correct answer)
- Directs air between the floor and defrost ducts
- Controls refrigerant flow to the evaporator
Correct answer: Controls coolant flow to the heater core
The heater control valve opens or restricts coolant flow to the heater core to control heat output.
ASE Heating and Cooling 3
The engine is at normal temperature and both heater hoses are hot, but the heater only blows cold air.
What is the most likely cause?