20 AC Diagnosis & Service Practice Questions & Answers

1. 1. A technician is diagnosing a split system with a thermostatic expansion valve on a cooling call. Measured superheat is 22°F, measured subcooling is 2°F, suction pressure is low and head pressure is low. The evaporator coil is clean and measured airflow is normal. What is the most likely cause?

   • A.The system is overcharged
   • B.The liquid-line filter-drier is restricted
   • C.The system is undercharged, most likely from a refrigerant leak✓ Answer
   • D.Indoor airflow across the evaporator is too low

   High superheat with low subcooling is the classic undercharge signature: there is not enough liquid in the system to back up in the condenser (so subcooling falls) and not enough liquid reaching the evaporator to keep it fed (so superheat climbs). A restriction would also raise superheat but would hold subcooling high, and low airflow would lower superheat. The correct follow-up is a leak search and repair, not simply adding refrigerant.

   Source: NATE Air Conditioning Service knowledge areas, refrigerant-system diagnostics (superheat and subcooling relationships)Report a problem with this question

2. 2. On a cooling call the technician measures 25°F superheat and 18°F subcooling. Suction pressure is low, head pressure is normal to slightly high, and the liquid line is noticeably cooler and lightly sweating immediately downstream of the filter-drier. What does this combination indicate?

   • A.A refrigerant undercharge
   • B.A compressor with worn valves
   • C.An overcharge of refrigerant
   • D.A restriction in the liquid line at the filter-drier✓ Answer

   High superheat together with high subcooling means liquid is backing up upstream of an obstruction while the evaporator is starved, which locates the fault between the condenser outlet and the metering device. A measurable temperature drop or sweating/frosting across the drier confirms the pressure drop is occurring there, and it is what separates a restriction from an undercharge, which would show low subcooling.

   Source: NATE Air Conditioning Service knowledge areas, refrigerant-circuit diagnostics (restriction versus undercharge)Report a problem with this question

3. 3. A fixed-orifice system is running with 3°F superheat and 18°F subcooling. Head pressure is high and compressor amp draw is above its nameplate rated load amps. The condenser coil is clean, the outdoor fan is moving full airflow, and outdoor ambient is moderate. What is the most likely cause?

   • A.The system has a refrigerant leak
   • B.The metering device is undersized
   • C.The indoor blower is running too fast
   • D.The system is overcharged with refrigerant✓ Answer

   Low superheat with high subcooling means excess liquid is stacking in the condenser and flooding the evaporator, which is the overcharge quadrant of the diagnostic matrix. Because the stem rules out the other cause of high subcooling and high head — poor condenser heat rejection — the extra refrigerant itself is the fault, and it also explains the elevated head pressure and current draw.

   Source: NATE Air Conditioning Service knowledge areas, refrigerant-system diagnostics (overcharge symptoms on fixed-orifice systems)Report a problem with this question

4. 4. A system has its charge verified by weight after a proper evacuation and its airflow verified. Running, it shows suction pressure higher than normal, head pressure lower than normal, superheat and subcooling both low, an evaporator temperature split of only 8°F, and compressor amp draw well below the nameplate rated load amps. What is the most likely fault?

   • A.The compressor is not pumping properly (worn valves or worn scroll flanks)✓ Answer
   • B.The liquid line is restricted
   • C.The system is undercharged
   • D.The condenser coil is dirty

   A compressor that cannot build a pressure differential lets suction rise and head fall at the same time, and because it moves less refrigerant it does less work, so amp draw drops below rated load and the temperature split collapses. Charge and airflow faults were eliminated first, and none of them produce high suction with low head and low amps simultaneously.

   Source: NATE Air Conditioning Service knowledge areas, system analysis (inefficient compressor symptoms)Report a problem with this question

5. 5. A technician finds the indoor coil completely iced. After thawing the coil and restarting the system, which reading best distinguishes low indoor airflow from a low refrigerant charge as the cause?

   • A.Discharge line temperature at the compressor
   • B.Suction pressure
   • C.Condenser fan motor amp draw
   • D.Liquid-line subcooling✓ Answer

   Both faults drive suction pressure down, so suction pressure alone cannot separate them; subcooling can, because a low charge leaves little liquid to stack in the condenser and reads low, while a low-airflow system still holds a full charge and reads normal or high subcooling with low superheat. Airflow should then be confirmed directly with a filter, coil and static-pressure check.

   Source: NATE Air Conditioning Service knowledge areas, symptom-driven diagnostics (frozen evaporator coil)Report a problem with this question

6. 6. On a no-cooling call the thermostat is calling, control voltage is present at the transformer secondary, 0 V is measured across the closed thermostat contacts, source voltage is measured across the two terminals of a pressure switch in the control string, and 0 V is measured across the contactor coil. What does this tell the technician?

   • A.That pressure switch is the open device in the control circuit✓ Answer
   • B.The control transformer has failed
   • C.The thermostat contacts are not closing
   • D.The contactor coil is shorted

   In a series control string, reading full source voltage across a device means the circuit is open at that device and the meter is completing the path, while 0 V across a switch means it is closed and passing current. Because the coil sees 0 V, the break is upstream of it, and the pressure switch showing the full drop is the open component; the technician must then find why the safety opened rather than jumper it.

   Source: NATE Air Conditioning Service knowledge areas, low-voltage control-circuit troubleshooting (hopscotch method)Report a problem with this question

7. 7. The outdoor fan runs normally, but the compressor hums for a few seconds and then trips on its internal overload. Line voltage measured at the load side of the contactor while the compressor attempts to start is within the nameplate range. What is the most appropriate next step?

   • A.De-energize and lock out the unit, safely discharge the run capacitor, and measure its capacitance before condemning the compressor✓ Answer
   • B.Condemn the compressor as seized and quote a replacement
   • C.Jumper the internal overload so the compressor can run long enough to test
   • D.Add refrigerant to raise suction pressure so the compressor can start

   A hum followed by an overload trip with correct supply voltage means the motor is drawing locked-rotor current, and an open or weakened run capacitor produces exactly that symptom in a permanent split capacitor compressor, so the cheap component must be proven good before the expensive one is condemned. The capacitor must be tested with the power off and the capacitor discharged, and an internal overload is a safety device that is never bypassed.

   Source: NATE Air Conditioning Service knowledge areas, electrical component checks (run capacitors and locked-rotor symptoms)Report a problem with this question

8. 8. A condensing unit has blown its fuses twice. With the power off, locked out, and the compressor leads disconnected, an ohmmeter reads continuity between a compressor terminal and the compressor shell. What is the correct course of action?

   • A.Replace the fuses and restart to see whether the fault repeats
   • B.Replace only the contactor, since the fuses cleared the fault
   • C.Treat it as a grounded (burned-out) compressor: replace it and perform a burnout cleanup, including new suction-line and liquid-line driers and an oil acid check✓ Answer
   • D.Install larger fuses so the unit will stay on line

   Continuity from a winding to the shell means the winding insulation has failed to ground, which is what blew the fuses and what will blow them again; the motor cannot be repaired in place. An electrical burnout also drives acid and contamination into the refrigerant and oil, so the system must be cleaned up with fresh driers and verified by acid test, and oversizing overcurrent protection to keep a faulted unit running is unsafe.

   Source: NATE Air Conditioning Service knowledge areas, repair and replacement (grounded compressor and burnout cleanup)Report a problem with this question

9. 9. With the power off and the compressor leads removed, a technician measures common-to-run at 1.9 ohms, common-to-start as infinite (no continuity), and run-to-start as infinite. No terminal reads continuity to the shell. What do these readings indicate?

   • A.The compressor is grounded
   • B.The start winding is open and the compressor must be replaced✓ Answer
   • C.The windings are normal and the fault is elsewhere
   • D.The run winding is shorted turn-to-turn

   In a healthy single-phase compressor the common-to-run resistance is the lowest, common-to-start is higher, and run-to-start equals the sum of the two; an infinite reading on both paths that include the start terminal means the start winding itself is broken. Because there is no continuity to the shell it is not a ground, and an open winding inside a hermetic shell cannot be repaired in the field.

   Source: NATE Air Conditioning Service knowledge areas, compressor winding continuity and ground testingReport a problem with this question

10. 10. A customer reports the outdoor unit never shuts off. With the thermostat set to OFF, the technician removes the low-voltage wire from the contactor coil and the compressor and outdoor fan keep running. What is the most likely fault and the immediate safe action?

    • A.The contactor power contacts are welded closed; open the outdoor disconnect to stop the unit, then replace the contactor✓ Answer
    • B.The transformer is backfeeding the coil; replace the transformer
    • C.The thermostat is miswired; rewire the thermostat and leave the contactor in service
    • D.The compressor is overcharged; recover refrigerant until it cycles off

    Removing the coil wire de-energizes the electromagnet, so if the load keeps running the line-voltage contacts are mechanically stuck or welded together and the control circuit no longer has any authority over the compressor. The unit must be stopped at the disconnect and locked out before work, and the damaged contactor replaced rather than filed or reused, because welded contacts leave the equipment without a means of control.

    Source: NATE Air Conditioning Service knowledge areas, electrical component checks (contactors and relays)Report a problem with this question

11. 11. A technician arrives to find a system locked out on a manual-reset high-pressure control. The customer is uncomfortable and asks to have the unit running before the technician leaves. What is the correct procedure?

    • A.Reset the control and leave, since it will trip again if a real problem exists
    • B.Jumper the high-pressure control so the unit will run, and return later to find the cause
    • C.Diagnose and correct the condition that caused the high pressure, then reset the control and verify normal operating pressures✓ Answer
    • D.Replace the high-pressure control, since a control that trips is defective

    A high-pressure control that has tripped is reporting an abnormal condition, not causing one, so the diagnosis has to move to what raised condensing pressure — restricted condenser airflow, a failed fan, overcharge or non-condensables. Bypassing or jumpering a safety control removes the protection that keeps the system from operating at destructive or hazardous pressures and is never an acceptable temporary fix.

    Source: NATE Air Conditioning Service knowledge areas, safety controls and service practice (high-pressure cutout)Report a problem with this question

12. 12. On a service call the contactor buzzes and chatters, repeatedly pulling in and dropping out, and the compressor makes a repeated starting noise. What should the technician measure first?

    • A.The refrigerant charge by weight
    • B.The insulation resistance of the compressor windings
    • C.The control voltage at the contactor coil while it attempts to pull in, looking for a drop below the coil's pick-up voltage✓ Answer
    • D.The temperature split across the evaporator

    Chattering means the coil is not holding the armature in, which happens when the voltage at the coil sags below its pick-up value under load — from a loose or corroded connection, an overloaded or failing transformer, or excessive resistance in a long control run. Measuring the coil voltage during the attempted pull-in, rather than at rest, is what exposes the sag, since a static reading can look normal.

    Source: NATE Air Conditioning Service knowledge areas, low-voltage circuit troubleshooting (contactor and transformer loading)Report a problem with this question

13. 13. A ceiling below an attic air handler is water stained. The technician finds standing water in the secondary drain pan and discovers that the condensate float safety switch has been jumpered out with a piece of wire. What is the correct response?

    • A.Leave the jumper in place so the customer keeps cooling, and clear the drain only
    • B.Replace the float switch with a jumper terminal block to prevent nuisance shutdowns
    • C.Drill a weep hole in the secondary pan so water can escape to the attic floor
    • D.Remove the jumper, restore the float switch to service, clear the plugged primary drain, and verify the switch shuts the system down✓ Answer

    Water in the secondary pan proves the primary drain has failed, and the float switch exists to shut the equipment down before that water reaches the finished space below; jumpering it removed the only protection and is what allowed the ceiling damage. The repair is to restore the safety, clear the blockage that caused the overflow, and prove by test that the switch actually interrupts the control circuit.

    Source: NATE Air Conditioning Service knowledge areas, condensate management and safety devicesReport a problem with this question

14. 14. An indoor blower section pulls air through the cooling coil, so the drain pan sits in negative pressure. The primary drain line is verified clear and correctly sloped, yet the pan overflows whenever the blower runs and drains normally when the blower is off. What is the most likely cause?

    • A.The evaporator coil is oversized for the blower
    • B.The drain line material is incompatible with condensate
    • C.The drain lacks a proper trap, so blower suction pulls air up the drain line and holds the water in the pan✓ Answer
    • D.The refrigerant charge is high, producing more condensate than the drain can carry

    On a draw-through arrangement the pan is below atmospheric pressure while the blower runs, so without a trap deep enough to hold a water seal against that negative pressure, air is drawn in through the drain outlet and the condensate cannot flow out. The symptom pattern — overflow only while the blower runs, free drainage when it stops — is the signature of a missing or too-shallow trap.

    Source: NATE Air Conditioning Service knowledge areas, condensate drainage on draw-through (negative pressure) drain pansReport a problem with this question

15. 15. After utility work on the building's three-phase service, a rooftop unit's scroll compressor runs loudly, draws current well below its rated load amps, and produces almost no difference between suction and discharge pressure. What is the fault and the correct correction?

    • A.The reversing valve is stuck mid-position; replace the valve
    • B.The compressor is running in reverse rotation; correct it by interchanging any two legs of the three-phase supply✓ Answer
    • C.The system is undercharged; add refrigerant until pressures separate
    • D.The compressor has failed internally; replace the compressor

    A three-phase scroll turned backwards cannot compress: the scrolls unmesh instead of sealing, so no pressure differential develops, the motor does little work and amp draw falls below rated load, and the machine is noisy. Because rotation direction on a three-phase motor is set by phase sequence, swapping any two supply legs reverses it, and the condition must be corrected quickly because prolonged reverse operation damages the scroll set.

    Source: NATE Air Conditioning Service knowledge areas, compressor diagnostics (scroll reverse rotation on three-phase power)Report a problem with this question

16. 16. A homeowner complains of a loud booming sound from the supply plenum each time the blower starts and each time it stops. Refrigerant pressures, superheat, subcooling and blower amp draw are all normal. What is the most likely cause?

    • A.A thermostatic expansion valve hunting
    • B.Air velocity noise at the supply diffusers
    • C.Large unsupported sheet-metal panels flexing as duct pressure changes (oil canning)✓ Answer
    • D.The compressor slugging liquid refrigerant on start-up

    A bang that occurs exactly at blower start and stop is tied to the pressure change in the duct, not to the refrigerant circuit, and a broad flat panel with no stiffening snaps between two shapes as pressure rises and falls. The cure is mechanical stiffening — cross-breaking, bracing or adding a support — and normal refrigerant-side readings rule out compressor or valve noise.

    Source: NATE Air Conditioning Service knowledge areas, noise and vibration analysis in air distribution systemsReport a problem with this question

17. 17. A system whose charge was just weighed in after a proper evacuation is running with high head pressure and high subcooling while superheat is normal. What should the technician investigate before touching the charge?

    • A.The thermostat heat anticipator setting
    • B.Condenser heat rejection — a dirty coil, a slow or failed condenser fan, or air recirculating back into the coil✓ Answer
    • C.The suction-line insulation
    • D.The indoor filter and blower speed

    Anything that keeps the condenser from rejecting heat raises condensing temperature and pressure and stacks liquid in the coil, which is why subcooling climbs even though the charge is correct — the same picture an overcharge makes. Since the charge was weighed in against a known line length, the airside of the condenser must be proven before refrigerant is removed, or a correctly charged system will be left short.

    Source: NATE Air Conditioning Service knowledge areas, refrigerant-system diagnostics (condenser airflow versus overcharge)Report a problem with this question

18. 18. Following a compressor change-out done in a hurry, a system runs with head pressure far higher than the outdoor ambient would produce, subcooling on the low side of normal, and a condensing saturation temperature far above the outdoor air temperature. The condenser is clean and the charge was weighed in. What is the most likely cause?

    • A.Non-condensable gases were left in the system because it was not evacuated deeply enough✓ Answer
    • B.The suction line is undersized
    • C.The system is overcharged with refrigerant
    • D.The metering device is undersized

    Non-condensables collect in the high side and add their own partial pressure, so head pressure and the pressure-derived saturation temperature read far above the temperature the condenser is actually operating at — the tell-tale gap between condensing saturation temperature and ambient. An overcharge would also raise head pressure but would push subcooling high, and the corrective action here is to recover, evacuate to a deep vacuum verified with a micron gauge, and recharge by weight.

    Source: NATE Air Conditioning Service knowledge areas, refrigerant-system diagnostics (non-condensables in the high side)Report a problem with this question

19. 19. The installation instructions on an air handler state a maximum total external static pressure of 0.5 in. w.c. The technician measures 0.95 in. w.c. total external static, with most of the reading appearing on the return side of the blower. What is the correct next diagnostic step?

    • A.Increase the blower speed until measured airflow reaches design
    • B.Measure the pressure drop across each return-side component — filter, return grille and return duct fittings — to isolate where the restriction is✓ Answer
    • C.Replace the blower motor, since it cannot overcome the duct system
    • D.Add refrigerant to compensate for the reduced airflow

    Total external static tells you a restriction exists but not where, so the diagnosis proceeds by measuring the drop across individual components until the offending one is found and corrected. Raising blower speed only masks the problem — it increases noise and power draw without fixing the restriction — and refrigerant charge is not a remedy for an airside fault.

    Source: NATE Air Conditioning Service knowledge areas, airside diagnostics and static pressure measurementReport a problem with this question

20. 20. While troubleshooting a condensing unit that has tripped its overcurrent protection twice, the technician opens the compressor terminal box and finds discolored, arced terminals, melted insulation and oil residue around the terminal plate. What is the correct action?

    • A.Keep the unit de-energized and locked out, do not attempt a restart, and repair or replace the compressor before the system is operated again✓ Answer
    • B.Replace the overcurrent protection with a higher rating so the unit stays on line
    • C.Re-energize briefly with the terminal cover off to watch which terminal arcs
    • D.Clean the terminals, reset the protection and run the unit to see whether it holds

    Arcing at a hermetic terminal plate with oil present is a terminal-venting hazard: a failing terminal can blow out under system pressure and release hot oil and refrigerant at the technician, so the equipment must stay off and locked out rather than be restarted to gather more evidence. Repeated tripping of overcurrent protection is a report of a real fault, and enlarging that protection to keep a damaged unit running removes the device that is limiting the damage.

    Source: NATE Air Conditioning Service knowledge areas, service safety and electrical troubleshooting (hermetic terminal failures)Report a problem with this question

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