HVAC Troubleshooting – System Diagnostics Practice Exam Updated
2026 | Complete Study Guide with Verified Questions and Detailed
Rationales Covering Refrigeration Cycle Diagnostics, Electrical Circuit
Testing and Fault Isolation, Thermostat and Control Systems
Troubleshooting, Airflow Measurement and Duct System Analysis,
Compressor and Condenser Faults, Heat Pump Operation and Defects,
Pressure and Temperature Readings, Refrigerant Issues and Leak
Detection, HVAC Safety Procedures, and Scenario-Based Questions for
HVAC Certification Exam Success
Question 1: A technician observes that the suction line is sweating heavily all the
way back to the compressor, and the compressor housing is extremely cold. What
is the most likely diagnosis?
A. Low refrigerant charge
B. Restricted metering device
C. Overcharge of refrigerant
D. Dirty condenser coil
CORRECT ANSWER: C. Overcharge of refrigerant
RATIONALE: Heavy sweating on the suction line extending to the compressor and a
cold compressor housing indicate liquid refrigerant is returning to the compressor
(floodback). This is a classic symptom of an overcharged system, where excess
refrigerant fails to boil off completely in the evaporator. A low charge would result in a
warm or dry suction line. A restricted metering device would cause low suction pressure
and likely frost at the restriction, not floodback. A dirty condenser would raise head
pressure but typically results in higher superheat, not floodback.
Question 2: When measuring superheat on a fixed orifice (TXV-free) system, which
two measurements are required?
A. Suction pressure and liquid line temperature
B. Suction pressure and suction line temperature
C. Discharge pressure and discharge line temperature
D. Liquid line pressure and evaporator inlet temperature
CORRECT ANSWER: B. Suction pressure and suction line temperature
RATIONALE: Superheat is the difference between the actual temperature of the vapor
leaving the evaporator and the saturation temperature corresponding to the pressure in
the evaporator. To calculate this, one must measure the suction pressure (to find
saturation temperature via a PT chart) and the actual temperature of the suction line at
the same location. Liquid line measurements relate to subcooling, not superheat.
Question 3: A heat pump is operating in heating mode. The outdoor unit is frosted
over, but the defrost cycle does not initiate. Which component is most likely faulty?
,A. Reversing valve solenoid
B. Defrost control board or defrost thermostat
C. Indoor blower motor
D. Compressor start capacitor
CORRECT ANSWER: B. Defrost control board or defrost thermostat
RATIONALE: The defrost cycle is controlled by a defrost control board, often triggered
by a defrost thermostat (sensor) on the outdoor coil or a time/temperature algorithm. If
the coil is frosted but no defrost occurs, the control logic or the sensor detecting the ice
is failed. The reversing valve determines mode (heating/cooling), not defrost initiation.
The indoor blower and start capacitor affect operation but do not control the defrost
logic.
Question 4: What is the primary purpose of measuring subcooling in an HVAC
system with a Thermostatic Expansion Valve (TXV)?
A. To determine if the compressor is overheating
B. To verify the proper refrigerant charge
C. To check the efficiency of the evaporator fan
D. To measure the airflow across the condenser
CORRECT ANSWER: B. To verify the proper refrigerant charge
RATIONALE: In systems with a TXV, the valve maintains a constant superheat.
Therefore, superheat is not a reliable indicator of charge. Subcooling, however, varies
directly with the amount of refrigerant in the condenser. Measuring subcooling is the
standard method for charging and diagnosing charge levels in TXV systems.
Compressor overheating is checked via discharge temperature or amp draw. Airflow is
measured with an anemometer or static pressure gauge.
Question 5: A technician measures high head pressure and low suction pressure.
The compressor amp draw is lower than normal. What is the most probable cause?
A. Refrigerant overcharge
B. Dirty condenser coil
C. Restriction in the liquid line or metering device
D. Worn compressor valves
CORRECT ANSWER: C. Restriction in the liquid line or metering device
RATIONALE: A restriction (such as a clogged filter-drier or stuck TXV) prevents
refrigerant from flowing into the evaporator. This causes pressure to build up before the
restriction (high head) and drop after it (low suction). Because the compressor is
pumping less mass flow due to the starvation, the amp draw drops. An overcharge or
dirty condenser would typically cause high amps. Worn valves usually cause high
suction and low head pressure.
,Question 6: Which instrument is best suited for detecting non-condensable gases
(air) in a refrigeration system?
A. Manifold gauge set only
B. Thermometer and psychrometer
C. Pressure-temperature relationship analysis
D. Ultrasonic leak detector
CORRECT ANSWER: C. Pressure-temperature relationship analysis
RATIONALE: Non-condensables (like air) increase the total pressure in the condenser
without raising the saturation temperature corresponding to the pure refrigerant. By
comparing the actual condensing pressure to the expected saturation pressure for the
measured condensing temperature, a technician can identify if the pressure is
abnormally high for that temperature, indicating non-condensables. A leak detector
finds leaks, not air inside. Gauges alone don't distinguish air without temperature
correlation.
Question 7: In a split AC system, the indoor evaporator coil is freezing up. Which of
the following is NOT a common cause?
A. Low airflow due to a dirty air filter
B. Low refrigerant charge
C. High outdoor ambient temperature
D. Faulty indoor blower motor
CORRECT ANSWER: C. High outdoor ambient temperature
RATIONALE: Freezing of the evaporator coil is caused by the coil temperature dropping
below 32°F (0°C). This happens due to low heat load (low airflow from dirty filters or bad
motors) or low pressure (low charge). High outdoor ambient temperatures typically
cause high head pressure and high compression ratios, but they do not cause the
evaporator to freeze; in fact, they make it harder for the system to cool, potentially
leading to high superheat, not freezing.
Question 8: What is the expected voltage reading across a closed contactor coil
terminals if the control circuit is functioning correctly?
A. 0 volts
B. Line voltage (e.g., 24V AC)
C. Half line voltage
D. Infinite resistance
CORRECT ANSWER: B. Line voltage (e.g., 24V AC)
RATIONALE: For a contactor to pull in and stay closed, its coil must be energized. This
means the full control voltage (typically 24V AC in residential HVAC) must be present
across the coil terminals. If 0 volts were present, the coil would not be energized.
Infinite resistance is an ohmmeter reading, not a voltage reading.
, Question 9: A heat pump customer complains that the house is not getting warm
enough in heating mode. The outdoor unit is running, but the indoor air temperature
rise is only 5°F. The suction pressure is high, and the discharge pressure is low.
What is the likely issue?
A. Stuck reversing valve in cooling mode
B. Low refrigerant charge
C. Dirty indoor filter
D. Outdoor fan motor failure
CORRECT ANSWER: A. Stuck reversing valve in cooling mode
RATIONALE: If a heat pump is calling for heat but the pressures indicate cooling
operation (high suction/low head relative to heating expectations, or simply reversed
pressure profiles), the reversing valve may have failed mechanically or electrically in the
cooling position. In this state, the indoor coil acts as the evaporator (cooling the house),
resulting in a minimal temperature rise or even cooling. Low charge would show low
suction and low head. A dirty filter would cause low suction and freezing.
Question 10: When using a digital multimeter to check a run capacitor, which
setting should be used?
A. AC Voltage
B. DC Voltage
C. Ohms (Resistance)
D. Capacitance (MFD/µF)
CORRECT ANSWER: D. Capacitance (MFD/µF)
RATIONALE: To accurately test a capacitor's health, one must measure its capacitance
value in microfarads (MFD or µF) and compare it to the rating printed on the capacitor.
While ohms can show a short or open, it cannot verify if the capacitor holds the correct
charge capacity. Voltage settings measure potential difference, not component
integrity.
Question 11: What is the primary symptom of a restricted capillary tube?
A. High suction pressure and high head pressure
B. Low suction pressure and low head pressure
C. High suction pressure and low head pressure
D. Low suction pressure and high head pressure
CORRECT ANSWER: B. Low suction pressure and low head pressure
RATIONALE: A restriction in the capillary tube limits the flow of refrigerant into the
evaporator. This starves the evaporator (low suction) and reduces the amount of
refrigerant circulating through the system, leading to a buildup in the condenser initially
but ultimately resulting in low head pressure due to reduced mass flow and heat
rejection load on the condenser from the lack of circulation. Note: In early stages, head
2026 | Complete Study Guide with Verified Questions and Detailed
Rationales Covering Refrigeration Cycle Diagnostics, Electrical Circuit
Testing and Fault Isolation, Thermostat and Control Systems
Troubleshooting, Airflow Measurement and Duct System Analysis,
Compressor and Condenser Faults, Heat Pump Operation and Defects,
Pressure and Temperature Readings, Refrigerant Issues and Leak
Detection, HVAC Safety Procedures, and Scenario-Based Questions for
HVAC Certification Exam Success
Question 1: A technician observes that the suction line is sweating heavily all the
way back to the compressor, and the compressor housing is extremely cold. What
is the most likely diagnosis?
A. Low refrigerant charge
B. Restricted metering device
C. Overcharge of refrigerant
D. Dirty condenser coil
CORRECT ANSWER: C. Overcharge of refrigerant
RATIONALE: Heavy sweating on the suction line extending to the compressor and a
cold compressor housing indicate liquid refrigerant is returning to the compressor
(floodback). This is a classic symptom of an overcharged system, where excess
refrigerant fails to boil off completely in the evaporator. A low charge would result in a
warm or dry suction line. A restricted metering device would cause low suction pressure
and likely frost at the restriction, not floodback. A dirty condenser would raise head
pressure but typically results in higher superheat, not floodback.
Question 2: When measuring superheat on a fixed orifice (TXV-free) system, which
two measurements are required?
A. Suction pressure and liquid line temperature
B. Suction pressure and suction line temperature
C. Discharge pressure and discharge line temperature
D. Liquid line pressure and evaporator inlet temperature
CORRECT ANSWER: B. Suction pressure and suction line temperature
RATIONALE: Superheat is the difference between the actual temperature of the vapor
leaving the evaporator and the saturation temperature corresponding to the pressure in
the evaporator. To calculate this, one must measure the suction pressure (to find
saturation temperature via a PT chart) and the actual temperature of the suction line at
the same location. Liquid line measurements relate to subcooling, not superheat.
Question 3: A heat pump is operating in heating mode. The outdoor unit is frosted
over, but the defrost cycle does not initiate. Which component is most likely faulty?
,A. Reversing valve solenoid
B. Defrost control board or defrost thermostat
C. Indoor blower motor
D. Compressor start capacitor
CORRECT ANSWER: B. Defrost control board or defrost thermostat
RATIONALE: The defrost cycle is controlled by a defrost control board, often triggered
by a defrost thermostat (sensor) on the outdoor coil or a time/temperature algorithm. If
the coil is frosted but no defrost occurs, the control logic or the sensor detecting the ice
is failed. The reversing valve determines mode (heating/cooling), not defrost initiation.
The indoor blower and start capacitor affect operation but do not control the defrost
logic.
Question 4: What is the primary purpose of measuring subcooling in an HVAC
system with a Thermostatic Expansion Valve (TXV)?
A. To determine if the compressor is overheating
B. To verify the proper refrigerant charge
C. To check the efficiency of the evaporator fan
D. To measure the airflow across the condenser
CORRECT ANSWER: B. To verify the proper refrigerant charge
RATIONALE: In systems with a TXV, the valve maintains a constant superheat.
Therefore, superheat is not a reliable indicator of charge. Subcooling, however, varies
directly with the amount of refrigerant in the condenser. Measuring subcooling is the
standard method for charging and diagnosing charge levels in TXV systems.
Compressor overheating is checked via discharge temperature or amp draw. Airflow is
measured with an anemometer or static pressure gauge.
Question 5: A technician measures high head pressure and low suction pressure.
The compressor amp draw is lower than normal. What is the most probable cause?
A. Refrigerant overcharge
B. Dirty condenser coil
C. Restriction in the liquid line or metering device
D. Worn compressor valves
CORRECT ANSWER: C. Restriction in the liquid line or metering device
RATIONALE: A restriction (such as a clogged filter-drier or stuck TXV) prevents
refrigerant from flowing into the evaporator. This causes pressure to build up before the
restriction (high head) and drop after it (low suction). Because the compressor is
pumping less mass flow due to the starvation, the amp draw drops. An overcharge or
dirty condenser would typically cause high amps. Worn valves usually cause high
suction and low head pressure.
,Question 6: Which instrument is best suited for detecting non-condensable gases
(air) in a refrigeration system?
A. Manifold gauge set only
B. Thermometer and psychrometer
C. Pressure-temperature relationship analysis
D. Ultrasonic leak detector
CORRECT ANSWER: C. Pressure-temperature relationship analysis
RATIONALE: Non-condensables (like air) increase the total pressure in the condenser
without raising the saturation temperature corresponding to the pure refrigerant. By
comparing the actual condensing pressure to the expected saturation pressure for the
measured condensing temperature, a technician can identify if the pressure is
abnormally high for that temperature, indicating non-condensables. A leak detector
finds leaks, not air inside. Gauges alone don't distinguish air without temperature
correlation.
Question 7: In a split AC system, the indoor evaporator coil is freezing up. Which of
the following is NOT a common cause?
A. Low airflow due to a dirty air filter
B. Low refrigerant charge
C. High outdoor ambient temperature
D. Faulty indoor blower motor
CORRECT ANSWER: C. High outdoor ambient temperature
RATIONALE: Freezing of the evaporator coil is caused by the coil temperature dropping
below 32°F (0°C). This happens due to low heat load (low airflow from dirty filters or bad
motors) or low pressure (low charge). High outdoor ambient temperatures typically
cause high head pressure and high compression ratios, but they do not cause the
evaporator to freeze; in fact, they make it harder for the system to cool, potentially
leading to high superheat, not freezing.
Question 8: What is the expected voltage reading across a closed contactor coil
terminals if the control circuit is functioning correctly?
A. 0 volts
B. Line voltage (e.g., 24V AC)
C. Half line voltage
D. Infinite resistance
CORRECT ANSWER: B. Line voltage (e.g., 24V AC)
RATIONALE: For a contactor to pull in and stay closed, its coil must be energized. This
means the full control voltage (typically 24V AC in residential HVAC) must be present
across the coil terminals. If 0 volts were present, the coil would not be energized.
Infinite resistance is an ohmmeter reading, not a voltage reading.
, Question 9: A heat pump customer complains that the house is not getting warm
enough in heating mode. The outdoor unit is running, but the indoor air temperature
rise is only 5°F. The suction pressure is high, and the discharge pressure is low.
What is the likely issue?
A. Stuck reversing valve in cooling mode
B. Low refrigerant charge
C. Dirty indoor filter
D. Outdoor fan motor failure
CORRECT ANSWER: A. Stuck reversing valve in cooling mode
RATIONALE: If a heat pump is calling for heat but the pressures indicate cooling
operation (high suction/low head relative to heating expectations, or simply reversed
pressure profiles), the reversing valve may have failed mechanically or electrically in the
cooling position. In this state, the indoor coil acts as the evaporator (cooling the house),
resulting in a minimal temperature rise or even cooling. Low charge would show low
suction and low head. A dirty filter would cause low suction and freezing.
Question 10: When using a digital multimeter to check a run capacitor, which
setting should be used?
A. AC Voltage
B. DC Voltage
C. Ohms (Resistance)
D. Capacitance (MFD/µF)
CORRECT ANSWER: D. Capacitance (MFD/µF)
RATIONALE: To accurately test a capacitor's health, one must measure its capacitance
value in microfarads (MFD or µF) and compare it to the rating printed on the capacitor.
While ohms can show a short or open, it cannot verify if the capacitor holds the correct
charge capacity. Voltage settings measure potential difference, not component
integrity.
Question 11: What is the primary symptom of a restricted capillary tube?
A. High suction pressure and high head pressure
B. Low suction pressure and low head pressure
C. High suction pressure and low head pressure
D. Low suction pressure and high head pressure
CORRECT ANSWER: B. Low suction pressure and low head pressure
RATIONALE: A restriction in the capillary tube limits the flow of refrigerant into the
evaporator. This starves the evaporator (low suction) and reduces the amount of
refrigerant circulating through the system, leading to a buildup in the condenser initially
but ultimately resulting in low head pressure due to reduced mass flow and heat
rejection load on the condenser from the lack of circulation. Note: In early stages, head
