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NATE AC Service practice questions

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Question 1Air Conditioning Service

A technician measures a residential split system's supply air at 55°F and return air at 75°F. What is the temperature split?

  1. 30°F
  2. 130°F
  3. 20°F Correct answer.
  4. 10°F

Explanation

Temperature split is the difference between return air and supply air temperature. Subtracting supply from return gives 75 minus 55, which equals 20°F. A healthy split for a typical residential system is roughly 15 to 22°F depending on indoor humidity and airflow.

Question 2Air Conditioning Service

Which instrument is used to measure the superheat of a system operating with a fixed-orifice metering device?

  1. A combustion analyzer
  2. A vacuum micron gauge
  3. A manifold gauge set with a clamp-on thermometer Correct answer.
  4. A sling psychrometer

Explanation

Superheat is the difference between the actual suction line temperature and the saturation temperature at suction pressure. A manifold gauge set provides the suction pressure, and a clamp-on thermometer reads the suction line temperature. Subtracting saturation temperature from line temperature yields superheat.

Question 3Air Conditioning Service

What is the primary purpose of a condensate trap on an air handler's drain line?

  1. To increase the drain line slope
  2. To detect a clogged drain and shut down the unit
  3. To filter debris from the drain water
  4. To prevent air from being drawn through the drain and to allow condensate to flow out Correct answer.

Explanation

A condensate trap creates a water seal that blocks the negative pressure in the cabinet from pulling air through the drain opening. It also allows condensate to drain against that pressure. Without a trap, air movement can prevent drainage and cause overflow.

Question 4Air Conditioning Service

A blower motor's capacitor is bulging and reads well below its rated microfarad value. What is the correct service action?

  1. Add a second capacitor in parallel to reach the rated value
  2. Increase the blower speed tap to compensate
  3. Replace the capacitor with one of the same microfarad rating and voltage Correct answer.
  4. Clean the capacitor terminals and return the unit to service

Explanation

A bulged capacitor that reads far below its rated microfarad value has failed and must be replaced. The replacement must match the original microfarad rating and equal or exceed the voltage rating. Motor performance and amp draw depend on proper capacitance.

Question 5Air Conditioning Service

Which refrigerant is commonly used in new residential air conditioning systems today and has a lower global warming potential than R-410A?

  1. R-22
  2. R-134a
  3. R-12
  4. R-32 Correct answer.

Explanation

R-32 is increasingly used in new residential air conditioning equipment and has a lower global warming potential than R-410A. R-22 and R-12 are ozone-depleting refrigerants phased out of new equipment. R-134a is used mainly in other applications and is being phased down as well.

Question 6Air Conditioning Service

The purpose of evacuating a refrigeration system before charging is to:

  1. Remove moisture and non-condensable gases from the system Correct answer.
  2. Verify that the refrigerant charge is correct
  3. Cool the compressor before startup
  4. Raise the system pressure to test for leaks

Explanation

Evacuation lowers the system pressure so that moisture boils off and non-condensables are removed. A deep vacuum pulled to a low micron level and held confirms the system is dry and tight. Charging before proper evacuation traps moisture and air that damage the system.

Question 7Air Conditioning Service

During a cooling call, the indoor blower runs but the outdoor condenser fan and compressor do not start. Which component should the technician check first?

  1. The low-voltage control circuit and contactor coil Correct answer.
  2. The supply air filter
  3. The condensate pump
  4. The indoor blower capacitor

Explanation

When the indoor blower runs but the outdoor unit is silent, the low-voltage signal to the contactor is a prime suspect. Checking the thermostat call, control wiring, and contactor coil voltage isolates whether the problem is control-side or high-voltage side. This is the logical first diagnostic step.

Question 8Air Conditioning Service

What does a reading of 500 microns on a vacuum gauge indicate about a system after evacuation?

  1. The system still contains significant moisture
  2. The system is overcharged with refrigerant
  3. The system has a large refrigerant leak
  4. The system has reached a deep vacuum suitable for charging Correct answer.

Explanation

A vacuum of 500 microns is a deep vacuum indicating that moisture and non-condensables have been largely removed. Many manufacturers accept a decay test starting near 500 microns. Refrigerant should not be added until this level is achieved and held.

Question 9Air Conditioning Service

A technician is checking charge on a system with a thermostatic expansion valve. Which measurement combination best confirms proper charge?

  1. Compressor amp draw alone
  2. Suction pressure alone
  3. Subcooling at the liquid line and superheat at the evaporator outlet Correct answer.
  4. Condenser entering air temperature alone

Explanation

TXV systems are charged by subcooling because the valve maintains superheat over a wide range. Superheat at the evaporator outlet confirms the valve is controlling properly. Using both measurements together verifies charge and valve operation.

Question 10Air Conditioning Service

A fixed-orifice system shows a suction superheat of 2°F at design conditions. What is the most likely condition?

  1. Failed condenser fan
  2. Overcharge or an oversized orifice Correct answer.
  3. Undercharge
  4. Restricted liquid line

Explanation

Low superheat on a fixed-orifice system indicates too much refrigerant is entering the evaporator. This points to overcharge or an orifice that is too large for the application. Excess refrigerant can flood the evaporator and risk liquid slugging the compressor.

Question 11Air Conditioning Service

The suction line of a running system is frosted back to the compressor. Which condition is most consistent with this symptom?

  1. A restricted metering device or low evaporator load causing flooding Correct answer.
  2. High outdoor ambient temperature
  3. Low refrigerant charge with high superheat
  4. Excessive airflow across the evaporator

Explanation

Frost on the suction line means refrigerant is boiling below freezing and liquid is reaching the compressor. A restricted or oversized metering device or low load can cause flooding and frost. The technician should check superheat and airflow before adding charge.

Question 12Air Conditioning Service

A technician measures 12°F subcooling on a TXV system with a clean condenser and proper airflow. The manufacturer specifies 10°F subcooling. What action is appropriate?

  1. Add refrigerant to raise subcooling further
  2. Increase blower speed to lower subcooling
  3. Replace the TXV immediately
  4. Recover refrigerant to reduce subcooling toward the target Correct answer.

Explanation

Subcooling above the target with a clean condenser indicates excess refrigerant. Recovering a small amount and rechecking brings subcooling to the specified value. Charge adjustments should always be made in small increments with stable operating conditions.

Question 13Air Conditioning Service

What is the effect of a dirty evaporator coil on system performance?

  1. Increased compressor capacity
  2. Increased airflow and higher suction pressure
  3. Reduced airflow, lower suction pressure, and possible coil freeze-up Correct answer.
  4. Higher subcooling with no change in superheat

Explanation

A dirty evaporator coil restricts heat transfer and airflow, lowering the load on the refrigerant. Suction pressure drops and the coil temperature can fall below freezing, causing ice formation. Reduced capacity and possible liquid return are the results.

Question 14Air Conditioning Service

A compressor draws higher-than-rated current and the condenser coil is hot to the touch with high head pressure. Which cause should be investigated first?

  1. Insufficient condenser airflow or a dirty condenser coil Correct answer.
  2. A failed run capacitor
  3. Low indoor airflow
  4. A restricted liquid line drier

Explanation

High head pressure with a hot condenser points to poor heat rejection. A dirty condenser coil, failed fan, or recirculating air are common causes. Correcting condenser airflow usually restores normal head pressure and amp draw.

Question 15Air Conditioning Service

Which safety practice is required before opening a refrigerant circuit for repair?

  1. Disconnect the thermostat only
  2. Recover the refrigerant into an approved recovery cylinder Correct answer.
  3. Vent the refrigerant to the atmosphere slowly
  4. Add nitrogen to the system and release it indoors

Explanation

Refrigerant must be recovered using EPA-approved recovery equipment before the circuit is opened. Venting refrigerant to the atmosphere is prohibited for ozone-depleting and regulated refrigerants. Recovery protects the technician, the environment, and the system.

Question 16Air Conditioning Service

A heat pump in heating mode is operating in defrost. What happens to the outdoor fan during a typical defrost cycle?

  1. It reverses direction
  2. It is stopped so warm refrigerant can melt the frost Correct answer.
  3. It runs at full speed to melt the ice
  4. It runs at half speed

Explanation

During defrost the reversing valve switches to cooling mode so hot discharge gas flows through the outdoor coil. The outdoor fan is typically stopped to keep the coil warm and speed defrost. The auxiliary heat energizes indoors to prevent cold supply air.

Question 17Air Conditioning Service

A technician finds a PSC blower motor that hums but does not rotate, and the motor shaft turns freely by hand. What is the most likely cause?

  1. Seized motor bearings
  2. A clogged evaporator coil
  3. An open run capacitor or incorrect capacitance Correct answer.
  4. Low refrigerant charge

Explanation

A motor that hums without rotating but spins freely by hand is likely receiving no phase shift from a failed run capacitor. Checking capacitor microfarads against the rated value confirms the diagnosis. Replacing the capacitor with the correct rating restores rotation.

Question 18Air Conditioning Service

Which of the following best describes the function of the reversing valve in a heat pump?

  1. It changes the direction of refrigerant flow to switch between heating and cooling Correct answer.
  2. It modulates refrigerant flow to the indoor coil
  3. It bypasses the metering device during defrost
  4. It controls the compressor speed

Explanation

The reversing valve redirects refrigerant flow so the indoor coil becomes the condenser in heating and the evaporator in cooling. A solenoid pilot controls the valve position. This single component allows one system to provide both heating and cooling.

Question 19Air Conditioning Service

A technician reads 0 volts at the contactor coil while the thermostat is calling for cooling. What is the next logical step?

  1. Add refrigerant
  2. Replace the compressor
  3. Check for voltage at the thermostat terminals and trace the control wiring Correct answer.
  4. Replace the contactor

Explanation

Zero volts at the coil with a call for cooling means the signal is being lost upstream. Checking the thermostat terminals and tracing the low-voltage wiring locates the open circuit. This avoids replacing parts that are not proven faulty.

Question 20Air Conditioning Service

A technician is preparing to braze refrigerant lines. Which practice protects the system from oxidation inside the tubing?

  1. Use a higher acetylene flame
  2. Purge with dry nitrogen during brazing Correct answer.
  3. Wet the tubing with water
  4. Seal both ends before heating

Explanation

A slow flow of dry nitrogen displaces oxygen inside the tubing during brazing, preventing scale formation. Oxide flakes can clog driers, valves, and screens. Nitrogen purging is a standard installation and repair practice.

Question 21Air Conditioning Service

A customer reports that the system cools adequately but the indoor humidity stays high. Which condition most directly explains this?

  1. Excessive refrigerant charge
  2. Too high an airflow across the evaporator, reducing latent removal Correct answer.
  3. A restricted liquid line drier
  4. A failed condenser fan motor

Explanation

High airflow moves air across the evaporator too quickly for adequate moisture removal, so sensible cooling occurs but latent capacity drops. Reducing airflow to the manufacturer's specification improves dehumidification. Charge and restrictions affect capacity differently.

Question 22Air Conditioning Service

A system with a piston metering device shows high superheat, low suction pressure, and normal subcooling. What is the most likely explanation?

  1. Overcharge
  2. Excess indoor airflow
  3. A failed condenser fan
  4. A restriction in the evaporator or a starved evaporator Correct answer.

Explanation

Normal subcooling indicates the liquid line is adequately charged, while high superheat and low suction pressure point to a starved evaporator. A restriction in the evaporator circuit or a blocked distributor can cause this pattern. The technician should inspect the evaporator and distributor tubes.

Question 23Air Conditioning Service

A technician is commissioning a new split system and measures 18°F subcooling with a TXV rated for 10°F at the same conditions. The condenser is clean and airflow is verified. Which conclusion is best supported?

  1. The compressor is inefficient
  2. The system is undercharged
  3. The system likely contains excess refrigerant or a liquid line restriction Correct answer.
  4. The TXV is stuck wide open

Explanation

Subcooling well above the target with a clean condenser and verified airflow suggests excess refrigerant or a restriction that backs liquid up in the condenser. The technician should confirm the restriction possibility before recovering charge. A restriction can mimic overcharge by raising subcooling.

Question 24Air Conditioning Service

A heat pump in heating mode has a suction pressure that is lower than expected and the outdoor coil is heavily frosted but the unit does not enter defrost. Which component is the most likely cause?

  1. The reversing valve solenoid coil
  2. The liquid line filter drier
  3. The indoor blower capacitor
  4. The defrost control board or its sensors Correct answer.

Explanation

A unit that frosts heavily without initiating defrost points to the defrost control or its temperature and time sensors. The control decides when to start and terminate defrost based on coil temperature and accumulated runtime. Testing the sensors and board confirms the fault.

Question 25Air Conditioning Service

A technician finds that a system's suction pressure and head pressure are both lower than normal, superheat is high, and subcooling is low. Which condition best fits these readings?

  1. Failed indoor blower motor
  2. Low refrigerant charge Correct answer.
  3. Overcharge
  4. Restricted condenser airflow

Explanation

Low charge reduces the amount of refrigerant circulating, so both suction and head pressures fall. High superheat and low subcooling confirm the system is starved. Leak detection and repair should precede recharging.

Question 26Air Conditioning Service

A technician is measuring airflow on a residential air handler using a duct traverse. Which instrument is designed to measure both total pressure and static pressure so velocity can be calculated?

  1. A clamp-on ammeter
  2. A sling psychrometer
  3. A pitot tube with a differential pressure manometer Correct answer.
  4. An anemometer held at the return grille

Explanation

A pitot tube measures total pressure and static pressure, and the velocity pressure difference between them is used to calculate duct velocity. This is the standard method for a duct traverse in air conditioning service work.

Question 27Air Conditioning Service

During a cooling call, a technician measures the temperature difference between return air and supply air across the evaporator. Which range is generally considered acceptable for a properly operating residential system?

  1. 30°F to 40°F
  2. 2°F to 6°F
  3. 45°F to 55°F
  4. 16°F to 22°F Correct answer.

Explanation

A properly operating residential system typically shows a temperature split of roughly 16°F to 22°F across the evaporator coil. A much lower split suggests low charge, low airflow, or a coil problem, while a much higher split suggests low airflow.

Question 28Air Conditioning Service

Which refrigerant is commonly used in new residential air conditioning systems and has a lower global warming potential than R-410A?

  1. R-22
  2. R-32 Correct answer.
  3. R-134a
  4. R-12

Explanation

R-32 is a lower-GWP refrigerant increasingly used in new residential air conditioning equipment. R-22 and R-12 are ozone-depleting refrigerants being phased out, and R-134a has a higher GWP and is not typical for residential AC.

Question 29Air Conditioning Service

A technician is checking a condenser unit and finds the coil fins crushed and clogged with grass clippings. What is the most likely effect on system operation?

  1. Lower suction pressure with normal head pressure
  2. Lower head pressure and lower compressor current
  3. No measurable change in operating pressures
  4. Higher head pressure and higher compressor current Correct answer.

Explanation

A restricted condenser coil cannot reject heat efficiently, so head pressure rises and the compressor draws more current. Cleaning the coil restores normal heat rejection and reduces compressor load.

Question 30Air Conditioning Service

Which device is used to measure the superheat of refrigerant leaving the evaporator?

  1. A megohmmeter
  2. A manifold gauge set with a clamp-on thermometer Correct answer.
  3. A manometer
  4. A vacuum gauge

Explanation

Superheat is the difference between the actual suction line temperature and the saturation temperature at suction pressure. A manifold gauge set provides the pressure for saturation temperature and a clamp-on thermometer reads the line temperature.

Question 31Air Conditioning Service

A technician is preparing to recover refrigerant from a system before repair. Which practice is required to protect the environment and comply with regulations?

  1. Draining refrigerant into the condensate drain
  2. Releasing refrigerant into the return duct
  3. Using certified recovery equipment and storing refrigerant in a proper recovery cylinder Correct answer.
  4. Venting the refrigerant to the atmosphere if the charge is small

Explanation

Refrigerant must be recovered with certified equipment and stored in an approved recovery cylinder. Venting refrigerant to the atmosphere is prohibited for most refrigerants under clean air regulations.

Question 32Air Conditioning Service

A technician is checking the electrical circuit of a condensing unit and finds the compressor contactor pitted and burned. What is the most appropriate action?

  1. Increase the breaker size to compensate
  2. Bypass the contactor with a jumper wire
  3. Replace the contactor with one of the same rating Correct answer.
  4. File the contacts smooth and reuse the contactor

Explanation

Burned or pitted contactor contacts increase resistance and can cause compressor damage, so the contactor should be replaced with one of equal rating. Filing contacts only provides a temporary fix and bypassing is unsafe.

Question 33Air Conditioning Service

A technician is measuring the temperature of the liquid line leaving the condenser to determine subcooling. Which two measurements are required?

  1. Liquid line pressure and liquid line temperature Correct answer.
  2. Outdoor ambient temperature and compressor current
  3. Return air temperature and supply air temperature
  4. Suction pressure and suction line temperature

Explanation

Subcooling is the difference between the saturation temperature at liquid line pressure and the actual liquid line temperature. Both the pressure and temperature must be taken at the liquid line for an accurate reading.

Question 34Air Conditioning Service

A technician is diagnosing a system with a thermostatic expansion valve and finds the sensing bulb has lost its insulation and is exposed to warm air. What is the most likely effect?

  1. The valve will lock closed and stop all flow
  2. The valve will overfeed refrigerant and superheat will drop excessively
  3. The valve will underfeed refrigerant and superheat will rise Correct answer.
  4. The valve will remain unaffected because the bulb only monitors pressure

Explanation

The TXV sensing bulb must sense suction line temperature accurately to regulate the valve. If the bulb is exposed to warm ambient air, it senses a false high temperature and drives the valve toward a more closed position, raising superheat.

Question 35Air Conditioning Service

A technician is checking a heat pump in heating mode and observes that the indoor coil is cold while the outdoor coil is warm. Which component is functioning correctly to produce this condition?

  1. The defrost board
  2. The indoor blower motor
  3. The reversing valve Correct answer.
  4. The outdoor fan capacitor

Explanation

In heating mode the reversing valve routes hot discharge gas to the indoor coil and suction gas to the outdoor coil, making the indoor coil the condenser. The observed condition confirms the reversing valve is directing refrigerant properly.

Question 36Air Conditioning Service

A technician is measuring static pressure across an evaporator coil and finds a drop of 0.65 in. w.c., while the manufacturer specifies a maximum of 0.30 in. w.c. What is the most likely cause?

  1. A faulty thermostat
  2. A refrigerant overcharge
  3. An oversized blower motor
  4. A restricted or dirty evaporator coil Correct answer.

Explanation

A high pressure drop across the coil indicates excessive resistance to airflow, most often from a dirty or restricted coil. Refrigerant charge and thermostat operation do not directly cause a high coil pressure drop.

Question 37Air Conditioning Service

A technician is checking a system and finds the suction line sweating and cold but the suction pressure is normal and superheat is low. Which condition is most consistent with these readings?

  1. An overcharge or overfeeding metering device Correct answer.
  2. A restricted liquid line filter drier
  3. A refrigerant undercharge
  4. A failed condenser fan motor

Explanation

Low superheat with normal suction pressure suggests the evaporator is being overfed with refrigerant, often from an overcharge or a metering device feeding too much. Undercharge and liquid line restrictions typically raise superheat.

Question 38Air Conditioning Service

A technician is checking a compressor and finds the winding resistance between run and common is infinite. What does this indicate?

  1. The winding is open Correct answer.
  2. The compressor is operating normally
  3. The capacitor is weak
  4. The winding is shorted to ground

Explanation

Infinite resistance across a winding indicates an open circuit in that winding, meaning current cannot flow through it. A short to ground would instead show continuity between a terminal and the compressor shell.

Question 39Air Conditioning Service

A technician is diagnosing a system with a fixed orifice and finds high suction pressure, low superheat, and a frosted suction line at the evaporator outlet. Which condition best explains these readings?

  1. An overcharge of refrigerant Correct answer.
  2. Low indoor airflow
  3. A failed run capacitor
  4. A restricted suction line

Explanation

An overcharge with a fixed orifice raises suction pressure and reduces superheat, which can frost the suction line. Low airflow or a suction restriction would instead lower suction pressure and raise superheat.

Question 40Air Conditioning Service

A technician is checking a system and finds the compressor running but the indoor blower not operating, with the evaporator coil iced. What is the most likely result if the system continues to run?

  1. The compressor will run cooler and last longer
  2. The condenser will overheat and trip the breaker
  3. The thermostat will automatically correct the airflow
  4. Liquid refrigerant may flood back and damage the compressor Correct answer.

Explanation

Without indoor airflow, the evaporator cannot absorb heat and liquid refrigerant can return to the compressor, causing slugging and damage. The coil will ice further and system performance will degrade.

Question 41Air Conditioning Service

A technician is checking a heat pump and finds the outdoor fan not running during a defrost cycle while the compressor continues to operate. What is the expected behavior?

  1. The outdoor fan should run faster to speed defrost
  2. The compressor should stop while the fan continues
  3. The reversing valve should switch to cooling permanently
  4. The outdoor fan should stop to allow the coil to warm and melt frost Correct answer.

Explanation

During defrost the outdoor fan is typically stopped so the outdoor coil can warm quickly and melt frost. The compressor continues to run in cooling mode to supply hot gas to the outdoor coil.

Question 42Air Conditioning Service

A technician is checking a system and finds the liquid line warm and the sight glass showing bubbles. Which condition is most likely?

  1. A restricted condenser
  2. A failed indoor blower
  3. A refrigerant undercharge Correct answer.
  4. An overcharge of refrigerant

Explanation

Bubbles in a sight glass with a warm liquid line suggest the liquid line is not full of liquid, which is consistent with an undercharge. An overcharge or condenser restriction would typically raise head pressure without bubbles.

Question 43Air Conditioning Service

A technician is measuring the current draw of a compressor and finds it well below the nameplate rated load amps while the system is undercharged. What is the best explanation?

  1. The capacitor is boosting the motor
  2. The breaker is oversized
  3. The compressor is operating more efficiently than rated
  4. The compressor is moving less mass flow and doing less work Correct answer.

Explanation

An undercharged system moves less refrigerant mass flow, so the compressor does less work and draws less current. This is not improved efficiency; it reflects reduced capacity and potential overheating.

Question 44Air Conditioning Service

A technician is checking a system and finds the suction pressure and head pressure both higher than normal with high superheat and a very hot compressor. Which condition is most consistent with these readings?

  1. An overcharge combined with high indoor load Correct answer.
  2. A restriction in the liquid line
  3. A failed thermostat
  4. A refrigerant undercharge

Explanation

High suction and head pressures together suggest excess refrigerant or a high heat load on the evaporator. Overcharge plus high indoor load can raise both pressures, while undercharge or a liquid line restriction would lower them.

Question 45Air Conditioning Service

A technician is commissioning a new system and finds the airflow is 20 percent below the manufacturer's specified CFM. What effect does this have on the system?

  1. It increases the sensible capacity of the coil
  2. It lowers the temperature split and improves dehumidification
  3. It has no effect on system performance
  4. It raises the temperature split and reduces capacity Correct answer.

Explanation

Low airflow increases the temperature split because each cubic foot of air gives up more heat, but total capacity drops and the coil may ice. Correcting airflow restores rated performance.

Question 46Air Conditioning Service

A technician is checking a system with a thermostatic expansion valve and finds the valve hunting, with superheat swinging widely. Which condition is most likely?

  1. An oversized valve or a bulb poorly clamped to the suction line Correct answer.
  2. A failed compressor
  3. A dirty condenser coil
  4. A refrigerant overcharge

Explanation

TXV hunting often results from an oversized valve or a sensing bulb that is not securely clamped and insulated on the suction line. Proper bulb mounting and correct valve sizing stabilize superheat control.

Question 47Air Conditioning Service

A technician is diagnosing a heat pump and finds the suction pressure lower than expected with a heavily frosted outdoor coil in heating mode. Which condition best explains this?

  1. A restricted outdoor coil or low airflow across it Correct answer.
  2. A refrigerant overcharge
  3. A failed indoor blower
  4. A stuck reversing valve in cooling mode

Explanation

A restricted or frosted outdoor coil in heating mode reduces heat absorption, lowering suction pressure. Correcting outdoor airflow or defrost operation restores normal pressures.

Question 48Air Conditioning Service

A technician is checking a system and finds the suction pressure and head pressure both lower than normal with high superheat and a partially restricted liquid line filter drier. What is the best explanation?

  1. The restriction has no effect on system pressures
  2. The restriction only affects the electrical circuit
  3. The restriction increases refrigerant flow and raises head pressure
  4. The restriction reduces refrigerant flow to the evaporator, lowering both pressures Correct answer.

Explanation

A restricted filter drier limits liquid refrigerant reaching the metering device, so less refrigerant flows through the system. This lowers both suction and head pressures and raises superheat.

Question 49Air Conditioning Service

A technician is evaluating a system and finds the compressor current above rated load amps with high head pressure and a condenser coil that is clean. Which condition is most likely?

  1. A restricted evaporator coil
  2. A failed thermostat
  3. Non-condensable gases in the system or an overcharge Correct answer.
  4. A refrigerant undercharge

Explanation

High head pressure with high compressor current on a clean condenser suggests excess refrigerant or non-condensable gases that cannot be condensed normally. Recovering and recharging with proper evacuation resolves the issue.

Question 50Air Conditioning Service

A technician is checking a system and finds the evaporator coil partially iced while the suction pressure is low and superheat is high. Which condition is most consistent with these readings?

  1. A failed condenser fan motor
  2. A refrigerant undercharge or restricted metering device Correct answer.
  3. A dirty condenser coil
  4. A refrigerant overcharge

Explanation

Low suction pressure with high superheat indicates insufficient refrigerant reaching the evaporator, often from an undercharge or a restricted metering device. The starved coil can drop below freezing and ice.