NAPA - National Automotive Parts Association Certified Mechanic Heating and Air Conditioning Questions and Answers — Questions and Answers
Question 1: A customer complains their vehicle's A/C blows cool air while driving at highway speeds but blows warm air when stopped in traffic or at idle. Which of the following is the MOST likely cause?
- A faulty blend door actuator
- A low refrigerant charge
- An inoperative radiator/condenser cooling fan (Correct answer)
- A clogged orifice tube
Correct answer: An inoperative radiator/condenser cooling fan
The condenser requires airflow to dissipate heat from the refrigerant. At highway speeds, 'ram air' from the vehicle's movement is sufficient. At idle or low speeds, the electric or engine-driven cooling fan is critical for pulling air through the condenser. If the fan is inoperative, the A/C system's high-side pressure will rise, leading to poor cooling performance.
Question 2: What are the two primary functions of a receiver/drier in an A/C system that uses an expansion valve?
- To store refrigerant and remove moisture (Correct answer)
- To compress refrigerant and increase its pressure
- To meter refrigerant flow and sense temperature
- To cool the refrigerant and protect the compressor from liquid
Correct answer: To store refrigerant and remove moisture
The receiver/drier is located on the high-pressure side of the system. Its main roles are to act as a storage vessel for liquid refrigerant and to use a desiccant material to absorb any moisture that could be harmful to the A/C system components.
Question 3: When servicing an A/C system containing R-1234yf refrigerant, which of the following is a critical difference compared to servicing an older R-134a system?
- R-1234yf operates at significantly higher pressures.
- The system can be topped off with R-134a if needed.
- R-1234yf does not require the use of specific lubricating oils.
- R-1234yf is mildly flammable and requires dedicated, certified service equipment. (Correct answer)
Correct answer: R-1234yf is mildly flammable and requires dedicated, certified service equipment.
R-1234yf is classified as an A2L mildly flammable refrigerant. Due to this, EPA regulations and safety standards require the use of specific service equipment (e.g., SAE J2843 or J3030 compliant R/R/R machines) that is designed with spark-free components to handle it safely. Cross-contaminating refrigerants is illegal and harmful to the system.
Question 4: A vehicle's heater blows cold air, even when the engine is at full operating temperature. The technician confirms the coolant level is correct and feels the two heater hoses at the firewall. The inlet hose is hot, while the outlet hose is significantly cooler. What is the MOST likely cause?
- A stuck-open thermostat
- A clogged or restricted heater core (Correct answer)
- A faulty blend door actuator
- An inoperative blower motor
Correct answer: A clogged or restricted heater core
The temperature difference between the hot inlet hose and the cool outlet hose indicates that hot coolant is flowing TO the heater core but is not circulating THROUGH it effectively to transfer heat and warm the outlet hose. This points to an internal blockage or restriction within the heater core.
Question 5: A technician connects a manifold gauge set to an A/C system and observes that both the static and running pressures on the high and low sides are significantly lower than normal for the ambient temperature. This reading is a classic indicator of:
- An overcharged system
- A restriction in the high side
- A low refrigerant charge (Correct answer)
- A failing compressor
Correct answer: A low refrigerant charge
When the entire system is low on refrigerant, there is not enough substance to create the necessary pressures for operation. Both the low-side (suction) and high-side (discharge) pressures will read below their normal operating ranges. This is the most common symptom of a leak that has caused an insufficient refrigerant charge.
Question 6: In a vehicle with an Automatic Temperature Control (ATC) system, which sensor provides the primary input to the control module for maintaining the desired cabin temperature?
- In-cabin temperature sensor (Correct answer)
- Ambient air temperature sensor
- Sunload sensor
- Evaporator temperature sensor
Correct answer: In-cabin temperature sensor
The in-cabin temperature sensor provides the main feedback to the ATC control module, constantly measuring the air temperature inside the passenger compartment. The module compares this reading to the driver's set temperature and adjusts the blend door, blower speed, and compressor operation accordingly to maintain the desired setting.
