Question 1

Which refrigerant is currently the standard for new vehicles due to lower global warming potential?

Accepted Answer

R-1234yf is the current standard for new vehicles because it has a much lower global warming potential (GWP) compared to R-134a. However, it is flammable and requires special handling.

Question 2

What is the approximate normal low-side pressure for an R-134a system at idle on a 70°F day?

Accepted Answer

Normal low-side pressure for R-134a at idle is approximately 25-45 psi. This pressure varies with ambient temperature and system load.

Question 3

What is the typical normal high-side pressure for R-134a at idle on a 70°F ambient temperature?

Accepted Answer

Normal high-side pressure for R-134a at idle is 150-250 psi at 70°F ambient temperature. Higher ambient temperatures will increase this pressure.

Question 4

Which type of oil must be used with R-1234yf refrigerant?

Accepted Answer

POE (polyolester) oil must be used with R-1234yf refrigerant. Mixing oil types can damage the system and must be avoided.

Question 5

What is subcooling in an A/C system?

Accepted Answer

Subcooling is when liquid refrigerant is cooled below its condensing temperature. Normal subcooling is 10-20°F and is measured at the condenser outlet.

Question 6

What happens to A/C pressures when ambient temperature increases?

Accepted Answer

When ambient temperature increases, both high-side and low-side pressures increase. This is because the condenser cannot reject heat as effectively.

Question 7

Which refrigerant is a CFC that has been phased out?

Accepted Answer

R-12 is a CFC (chlorofluorocarbon) that was phased out due to its ozone-depleting potential. It cannot be used in new vehicles.

Question 8

Where is an accumulator located in an A/C system?

Accepted Answer

An accumulator is located on the low side of the system with orifice tube expansion devices. It prevents liquid refrigerant from entering the compressor.

Question 9

What is the purpose of the desiccant in a receiver/drier or accumulator?

Accepted Answer

The desiccant in a receiver/drier or accumulator absorbs moisture from the refrigerant. Moisture in the system can form acids and cause ice blockage.

Question 10

Which compressor type uses an internal control valve to adjust displacement?

Accepted Answer

Variable displacement compressors have an internal control valve that adjusts displacement based on suction pressure, improving efficiency and reducing cycling.

Question 11

What does low superheat in an evaporator indicate?

Accepted Answer

Low superheat indicates that liquid refrigerant is reaching the compressor, which can cause damage. Superheat should be 8-12°F for TXV systems.

Question 12

Which expansion device maintains a constant superheat of 8-12°F?

Accepted Answer

The thermostatic expansion valve (TXV) senses the evaporator outlet temperature and maintains a constant superheat of 8-12°F.

Question 13

Where is the condenser typically mounted in a vehicle?

Accepted Answer

The condenser is mounted in front of the radiator where it can reject heat to the surrounding air using the cooling fan.

Question 14

What is the normal air gap specification for a compressor electromagnetic clutch?

Accepted Answer

The compressor clutch air gap should be 0.014-0.031 inches. If the gap is too large, the clutch won't engage; if too small, it will slip.

Question 15

What does a thermostatic switch on an evaporator prevent?

Accepted Answer

A thermostatic switch prevents the evaporator from freezing by cutting off the compressor clutch when evaporator temperature drops below freezing.

Question 16

Which refrigerant type is an HFC that was commonly used from 1994-2017?

Accepted Answer

R-134a is an HFC (hydrofluorocarbon) that became the standard refrigerant from 1994 until R-1234yf replaced it as the newer standard.

Question 17

What is required before performing A/C system service?

Accepted Answer

EPA Section 609 certification requires that refrigerant be recovered from the system before any A/C service is performed. Refrigerants cannot be vented.

Question 18

A vehicle's A/C system shows high-side pressure of 350 psi on a 70°F day with the compressor running. What is the most likely cause?

Accepted Answer

High-side pressure of 350 psi (normal is 150-250 psi) indicates refrigerant overcharge. Air in the system or condenser airflow restriction could also cause this.

Question 19

A customer reports weak cooling that improves when driving at highway speeds. What should be checked first?

Accepted Answer

Weak cooling that improves at highway speeds suggests the electric condenser fan isn't operating properly at idle, allowing better airflow at speed.

Question 20

What should be done before adding refrigerant to an existing R-134a system?

Accepted Answer

Before adding refrigerant, the existing refrigerant must be recovered and recycled using EPA-certified equipment to prevent contamination and environmental damage.

Question 21

A technician measures 15°F of superheat in the evaporator outlet. What does this indicate?

Accepted Answer

15°F of superheat is higher than the normal 8-12°F for TXV systems, indicating the TXV may not be sensing temperature properly or the system is undercharged.

Question 22

A scan tool shows high-side pressure of 180 psi and low-side pressure of 55 psi during operation. What is most likely?

Accepted Answer

High-side pressure of 180 psi (low for normal) and low-side pressure of 55 psi (high for normal) indicates the expansion device is restricted, causing high evaporator pressure.

Question 23

Which issue would cause a compressor clutch to not engage despite adequate refrigerant charge?

Accepted Answer

A low-side pressure cutoff switch prevents clutch engagement when pressure is too low to protect the compressor from damage and oil starvation.

Question 24

A refrigerant leak is suspected. Which method is best for locating a small leak on an R-134a system?

Accepted Answer

An electronic leak detector is most effective for finding small refrigerant leaks. UV dye can also be used. Nitrogen should NOT be used due to explosion risk.

Question 25

What is the purpose of recirculation mode in the HVAC system?

Accepted Answer

Recirculation mode uses interior air instead of outside air, allowing faster cooling and reducing entry of outside odors and contaminants.

Question 26

A heater outputs only warm air instead of adjusting temperature. What is the likely cause?

Accepted Answer

A stuck blend door (temperature control door) prevents mixing of heated and cooled air, resulting in one-temperature output.

Question 27

When replacing a scroll compressor with R-134a, which oil quantity should be used?

Accepted Answer

The oil quantity must be matched to the specific compressor and system. Adding too much or too little oil can cause performance problems and damage.

Question 28

What is the primary disadvantage of R-1234yf compared to R-134a?

Accepted Answer

R-1234yf is slightly flammable (Class 2L), requiring different handling procedures and equipment compared to non-flammable R-134a.

Question 29

A blower motor only works on high speed. What is the most common cause?

Accepted Answer

When a blower only works on high speed, the series resistor pack (used for analog speed control) has typically failed, bypassing low and medium speeds.

Question 30

A MOSFET blower control module has failed. Which speed control method will be lost?

Accepted Answer

A MOSFET module provides PWM (pulse width modulation) control for smooth variable blower speeds. Failure results in either no blower or maximum speed.

Question 31

A cabin air filter is clogged. What symptom would be most noticeable?

Accepted Answer

A clogged cabin air filter reduces airflow from HVAC vents, affecting both heating and cooling output and comfort.

Question 32

What indicates that a temperature control actuator has failed?

Accepted Answer

A failed actuator prevents the blend door from moving, resulting in no response to temperature control adjustments on the climate control panel.

Question 33

A heater core has a restriction. Which symptom is most likely?

Accepted Answer

A restricted heater core reduces coolant flow through it, resulting in weak heat output from the cabin vents.

Question 34

Why must PAG and POE oils never be mixed in an A/C system?

Accepted Answer

PAG and POE oils can chemically react and form acids and sludge, damaging the compressor and other system components.

Question 35

An A/C system uses an orifice tube. Where must an accumulator be located?

Accepted Answer

Orifice tube systems require an accumulator on the low side to prevent liquid refrigerant from entering the compressor inlet.

Question 36

What condition requires a NEW receiver/drier to be installed during system service?

Accepted Answer

The receiver/drier's desiccant becomes saturated with moisture when the system is opened. It must be replaced to prevent moisture-related damage.

Question 37

An electric compressor on a hybrid vehicle differs from a traditional compressor because it:

Accepted Answer

Electric compressors on hybrid and EV vehicles are driven by electric motors, allowing operation independent of engine speed.

Question 38

A condenser fan is engine-driven. When should it operate at maximum speed?

Accepted Answer

Engine-driven condenser fans operate at maximum speed when needed most—at idle with high ambient temperature—to reject heat when airflow is minimal.

Question 39

Refrigerant exits the evaporator as a:

Accepted Answer

Refrigerant exits the evaporator as a vapor with superheat (8-12°F above saturation temperature for TXV systems) to protect the compressor.

Question 40

A receiver/drier is used with which type of expansion device?

Accepted Answer

A receiver/drier is used on the high side of TXV systems to store liquid refrigerant and remove moisture.

Question 41

What is the result of moisture contamination in an A/C system?

Accepted Answer

Moisture in A/C systems combines with refrigerant and oil to form acids that damage compressors and other components, and can freeze at expansion devices.

Question 42

A scroll compressor differs from a reciprocating piston compressor by having:

Accepted Answer

Scroll compressors use two spiral-shaped scrolls for smoother compression with no intake/discharge valves, resulting in quieter operation.

Question 43

A system shows low-side pressure of 15 psi and high-side pressure of 180 psi. Low-side pressure increases when the compressor clutch is cycled off. What is the most likely cause?

Accepted Answer

Low pressure increasing when clutch cycles off indicates the compressor isn't pumping—pressure equalizes through the non-moving compressor. A restricted TXV/orifice would show continuous high pressure differential.

Question 44

A vehicle has been retrofitted from R-12 to R-134a. Which precaution is critical before adding refrigerant?

Accepted Answer

Retrofitting requires verifying seal compatibility, checking for mineral oil contamination (incompatible with PAG), and ensuring adequate desiccant for moisture removal in a converted system.

Question 45

A technician measures subcooling of 5°F at the condenser outlet. What does this indicate?

Accepted Answer

Subcooling of 5°F is lower than the normal 10-20°F, indicating the system is undercharged. The condenser outlet is receiving insufficiently liquid.

Question 46

Explain why orifice tube systems do not provide superheat control.

Accepted Answer

Orifice tubes provide a fixed restriction with no feedback mechanism, unlike TXVs which sense evaporator temperature and adjust opening to maintain constant superheat.

Question 47

A scan tool shows high refrigerant pressure at the compressor inlet despite adequate charge. The low-side line is very cold. What is happening?

Accepted Answer

A severely restricted expansion device prevents refrigerant flow to the evaporator, causing high suction-side pressure and evaporator outlet temperature to drop below freezing.

Question 48

A variable displacement compressor cycles on and off frequently when the system is properly charged. What is the most likely cause?

Accepted Answer

Variable displacement compressors modulate stroke to maintain minimum suction pressure. Frequent cycling indicates the suction pressure is exceeding the control threshold, preventing displacement reduction.

Question 49

When retrofitting a system from R-12 to R-1234yf, which components must definitely be changed?

Accepted Answer

R-1234yf requires POE oil (not mineral or PAG), updated receiver/drier, and many systems require compressor compatibility verification due to different thermal/pressure characteristics.

Question 50

A compressor clutch engages but immediately disengages after 2-3 seconds during operation. The high-pressure cutoff switch setting is 400 psi. What is the likely fault?

Accepted Answer

Rapid clutch cycling indicates high-side pressure is spiking above the 400 psi cutoff, preventing compressor operation. Causes include condenser airflow blockage or system overcharge.

Question 51

A heater has no output in defrost mode but works fine in other modes. Which component should be diagnosed first?

Accepted Answer

Defrost mode uses a dedicated blend door position. If heating works in other modes, the heater core and water flow are functional, pointing to the defrost blend door actuator.

Question 52

A blower motor has been replaced but only operates on high speed with a MOSFET controller. What is the problem?

Accepted Answer

If the new motor only works on high with a MOSFET, the control signal is not varying—either the MOSFET is stuck or the PCM PWM signal is always at 100%.

Question 53

A service manual specifies 3.0 oz of PAG-46 oil for a compressor replacement. A technician added 4.0 oz instead. What problems could result?

Accepted Answer

Excess oil foams and fails to properly coat bearing surfaces, reduces heat transfer in the compressor, and can reduce cooling performance significantly.

Question 54

A UV dye test shows dye at a compressor shaft seal, but pressure gauges show stable readings. What should be recommended?

Accepted Answer

Any dye visible at the compressor seal indicates oil/refrigerant is escaping. Even small leaks will eventually cause loss of charge and compressor lubrication failure.

Question 55

A technician is converting an R-134a system to R-1234yf. The system contains PAG-46 oil. What is the critical step that must be performed?

Accepted Answer

Complete system flushing and desiccant replacement are necessary to remove all PAG oil before adding R-1234yf with POE oil, since these oils cannot be mixed.

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