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These 10 free NATE-AC-SERVICE questions are organized by exam domain, so you can see how each part of the Air Conditioning Service Certification blueprint is tested. Reveal the answer and explanation under each question.
Domain 1: Installation
Question 1
During startup of a draw-through air handler, water accumulates in the pan while the blower runs but drains promptly when it stops. The pan is level as required, and the drain is clear with the specified downward slope. The pipe runs directly from the negative-pressure pan connection to an open outlet without a water seal. Which installation change addresses the blower-dependent drainage problem?
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Correct answer: A - Install and prime a trap sized for the negative pan pressure.
Question 2
After replacing a leaking coil, a technician passes the specified nitrogen pressure test and then evacuates the repaired circuit. The equipment instructions require reaching 500 microns or lower, isolating the pump for 10 minutes, and finishing at 1,000 microns or lower with a stable reading. A gauge on the system side reaches 420 microns. After isolation it rises to 780 microns and remains there through the end of the test. What does the standing-vacuum test establish?
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Correct answer: B - The equipment’s specified evacuation test has been passed.
Domain 2: Service
Question 3
A split system with a TXV is cooling poorly. After operation stabilizes, evaporator-outlet superheat is 30°F and condenser-outlet subcooling is 20°F; both normally run near 10°F under these conditions. Indoor airflow and condenser airflow meet specifications. Insulated contact probes show an 8°F temperature decrease across the liquid-line filter-drier, and pressure measurements confirm an excessive drop across that same component. Which fault best fits the complete set of readings?
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Correct answer: C - A restriction in the liquid-line filter-drier
Question 4
During a qualified technician’s energized diagnostic test, a two-pole contactor is pulled in and the compressor is running. Incoming voltage is 240 VAC, but the compressor receives only 222 VAC. The closed L1-T1 contact measures 17.8 VAC across it; the closed L2-T2 contact measures 0.2 VAC. Connections outside the contactor show negligible voltage drop. Where is the voltage being lost?
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Correct answer: D - Excessive resistance at the closed L1-T1 contacts
Question 5
An overheated single-phase hermetic compressor will not restart. With all power isolated and external wires removed, C-R and C-S both read OL, while R-S reads 6 Ω. No low-resistance path is found from any terminal to the shell. The compressor’s internal thermal protector is in the common lead and resets automatically after cooling. The next diagnostic step is to:
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Correct answer: A - Allow it to cool while isolated, then repeat the winding tests.
Question 6
A 3-ton air handler needs 1,200 CFM for the current cooling setup and is already on its highest blower-speed setting. At the manufacturer’s specified external-static test points, supply pressure is +0.30 in. w.c. and return pressure is −0.50 in. w.c. The high-speed blower table lists 1,300 CFM at 0.30 in. w.c., 1,200 CFM at 0.50 in. w.c., and 1,000 CFM at 0.80 in. w.c. The coil and correct filter are clean. Which assessment is supported by these measurements?
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Correct answer: A - 0.80 in. w.c. and 1,000 CFM; reduce external airflow resistance.
Domain 3: System Components
Question 7
A condenser-fan PSC motor requires a 5-µF continuous-duty run capacitor rated at least 370 VAC. Its capacitor has failed. The service truck has the four parts listed below, all with compatible mounting and connections. Which replacement preserves the specified capacitance and voltage withstand rating?
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Correct answer: D - A 5-µF, 440-VAC motor run capacitor
Question 8
A TXV sensing bulb has come loose from the evaporator-outlet suction line and is hanging in warm return air. Adequately subcooled liquid reaches the valve, and the external equalizer is unobstructed and correctly connected. Relative to the properly mounted bulb, what change in valve operation would this fault tend to produce?
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Correct answer: D - More bulb pressure opens the valve and lowers outlet superheat.
Domain 4: Applied Knowledge
Question 9
At the outlet of an evaporator using a zeotropic refrigerant blend, the suction-line temperature is 55°F. At the pressure measured at that same location, the refrigerant chart lists a dew-point temperature of 43°F and a bubble-point temperature of 38°F. The equipment’s acceptable evaporator-outlet superheat range is 10-14°F. How should the technician interpret this reading?
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Correct answer: B - 12°F, within the specified superheat range
Question 10
The disconnect for a variable-speed condensing unit has been locked out. Its service instructions require waiting five minutes and verifying that the inverter DC bus is discharged before terminal work. After five minutes, a correctly rated meter still reads 235 VDC across the bus. Before touching the drive terminals, the technician must:
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Correct answer: C - Maintain isolation and follow the approved stored-energy discharge procedure.
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