Eight free practice questions on diagnosis, refrigerant-circuit repairs, recovery and decommissioning, for skill set D (Service) of Ontario's 313D Residential Air Conditioning Systems Mechanic Certificate of Qualification exam, which Skilled Trades Ontario (STO) sets and delivers. 313D is an Ontario trade with its own exam, separate from the 313A Refrigeration and Air Conditioning Systems Mechanic. These are original questions written by TicketPrep from public sources, Ontario regulations and the trade's working practice, not questions from the exam. Each has four options and one correct answer, as every question in our bank does. Answer each one before opening the explanation. No account, no email.
01The questions
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A two-year-old R-410A heat pump with a TXV has cooled weakly since it was installed. Superheat is high and subcooling is 2 K against the maker's 6 K. Today's leak test finds no leaks. The nameplate charge covers 8 m (26 ft) of line set and the manual adds 15 g per metre (0.16 oz per foot) beyond that; this line set is 20 m (66 ft). The install sheet records the factory charge only. Which cause fits?
- A leak too small for the test has drained it.
- The line-set adjustment was never weighed in.
- The TXV is underfeeding the indoor coil.
- Non-condensables are holding the subcooling down.
Show the answer
B. Correct. The extra line holds refrigerant the factory charge does not cover: (20 − 8) × 15 g = 180 g. The system has run short since day one, holds tight today, and the record shows only the factory charge, so the adjustment was missed. With today's no-leak notice in place, weigh in 180 g and confirm subcooling reaches the maker's 6 K.
Why not the others
A. A leak shows as performance that falls off over time, and today's test found none. This system has been short since it was installed, and the install sheet shows the adjustment for the extra 12 m was never added.
C. An underfeeding valve leaves liquid backed up in the condenser, so subcooling would be high. Subcooling of 2 K against a 6 K target says the condenser is short of liquid.
D. Non-condensables raise the head pressure and give a high subcooling reading, not a low one. Low subcooling with high superheat is a shortage of refrigerant.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-14.02); O. Reg. 463/10
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An R-22 condensing unit removed from a house has had its refrigerant recovered and is going to a scrap dealer. Under O. Reg. 463/10, what must be on it before it may be dismantled or scrapped?
- A notice that it holds no refrigerant, with your ODP details.
- A leak-test notice showing no leaks, dated within six months.
- A label giving the refrigerant type and the amount recovered.
- A copy of the record of refrigerant added over its life.
Show the answer
A. Correct. Under s. 32 the certified person who finds that the equipment holds no refrigerant affixes a notice giving the date, their name, ODP certificate number and expiry, the employer and a statement that it contains no refrigerant, and keeps a copy for two years. Under s. 41(1) it may not be dismantled, destroyed or landfilled without that notice.
Why not the others
B. The six-month leak-test notice is the condition for refilling equipment with refrigerant (s. 25(1)), not for disposing of it. Disposal needs the notice that the unit holds no refrigerant.
C. The type and an estimate of the amount recaptured go into the service records the employer keeps (s. 23(1)), not on a label for the scrap dealer.
D. Service records are kept by the employer for two years; they are not attached to the unit. What the unit must carry is the no-refrigerant notice.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-15.02); O. Reg. 463/10
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A recovery cylinder in the shop has lost its label. After a night at 20 °C (68 °F) its gauge reads 1 360 kPa (197 psig). At 20 °C the P/T chart gives R-22 809 kPa (117 psig), R-410A 1 346 kPa (195 psig, bubble) and R-32 1 373 kPa (199 psig). What does the reading tell you about the contents?
- R-32, because the reading is close to its P/T value.
- R-410A, because the reading is close to its P/T value.
- R-410A or R-32, because their P/T values are this close.
- R-22 with air, because the reading is above its P/T value.
Show the answer
C. Correct. At 20 °C R-410A and R-32 differ by only 27 kPa (about 4 psi), which is inside what a field gauge and a small temperature error can hide, and a mixture of the two would read in the same band. Treat the cylinder as unidentified: confirm its contents with a refrigerant identifier, and add nothing to it until then.
Why not the others
A. The reading sits between R-32's 1 373 kPa (199 psig) and R-410A's 1 346 kPa (195 psig). Pressure alone cannot tell which, or whether it is a mixture.
B. The reading is close to R-410A's 1 346 kPa (195 psig), but it is just as close to R-32's 1 373 kPa (199 psig). Pressure alone cannot separate them.
D. R-22 at 20 °C is 809 kPa (117 psig); it would take about 550 kPa (80 psi) of air to reach this reading. A reading that matches two other refrigerants this closely points to one of them, but cannot say which.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-16.01)
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The scroll compressor of a 3-ton fixed-orifice R-410A split in a bungalow has failed mechanically: its windings test sound, and the oil drained from it is clean and passes the acid test. Past tickets record two iced indoor coils, and today the filter is collapsed and the coil face is matted with dust. What must be corrected along with the compressor change?
- The piston size, by fitting a larger one to pass more refrigerant.
- The off-cycle migration, by adding a crankcase heater to the unit.
- The acid in the system, by a full burnout clean-up with driers.
- The indoor airflow, by clearing the coil and fitting a new filter.
Show the answer
D. Correct. Starved airflow cuts the load on a fixed-orifice coil, so the piston feeds more liquid than the coil can boil: superheat falls to zero, the coil ices and liquid returns to the compressor, washing oil off the scrolls and bearings. Clean oil and sound windings point to that mechanical wear, not a burnout. Restore airflow or the new compressor fails the same way.
Why not the others
A. A larger piston feeds even more liquid into a coil that already cannot boil what it gets, making the flood-back worse. The fix is restoring indoor airflow.
B. Migration does flood compressors during the off cycle, but the history here is iced coils with a dirty filter and coil, a running condition. A heater alone leaves the starved coil sending liquid back on every cycle.
C. A burnout clean-up treats acid and carbon from a motor failure. The oil is clean, it passes the acid test and the windings are sound, so there is no burnout; the cause is liquid returning from a starved coil.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-16.02)
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To change the TXV on a two-storey house's attic coil, you want to pump the R-410A charge into the outdoor unit, whose manual says it can store at most 3.60 kg (7.94 lb). The install sheet records the 3.40 kg (7.50 lb) factory charge plus 0.62 kg (1.37 lb) added for the line set, and the charge is unchanged since. What is the least you must recover into a cylinder before pumping the rest down?
- None: pump the whole charge down.
- 0.20 kg (0.44 lb), then pump the rest down.
- 0.42 kg (0.93 lb), then pump the rest down.
- 0.62 kg (1.37 lb), then pump the rest down.
Show the answer
C. Correct. The system holds 3.40 + 0.62 = 4.02 kg (8.86 lb) and the outdoor unit can store 3.60 kg, so 4.02 − 3.60 = 0.42 kg (0.93 lb) goes into a recovery cylinder first. Pumping more into the unit than it can hold fills the condenser with liquid: the head pressure climbs fast, and liquid shut in with no vapour space can build very high pressure as it warms.
Why not the others
A. This takes the system as holding only its 3.40 kg factory charge, which fits under 3.60 kg. The 0.62 kg added for the line set is in the system too: 4.02 kg in all, 0.42 kg more than the outdoor unit can store.
B. This is the room left above the factory charge, 3.60 − 3.40 = 0.20 kg, not the amount over the limit. Recovering 0.20 kg still leaves 4.02 − 0.20 = 3.82 kg, 0.22 kg more than the unit can store.
D. This recovers the whole line-set addition, which is more than the least. The outdoor unit can store 3.60 kg, so only 4.02 − 3.60 = 0.42 kg has to come out first, 0.20 kg less than this.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-16.03)
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You will search a central air conditioner's indoor coil and suction line for a slow leak on its operating charge. Stopped and left to stand, the low side reads 1 510 kPa (219 psig) in the 24 °C (75 °F) house; running in cooling, suction is 860 kPa (125 psig). When are those low-side parts searched?
- Running in cooling, while the coil is cold and sweating.
- Running, in the first minute after the compressor starts.
- Off, once the pressures have equalized and settled.
- Running in cooling, with the indoor blower switched off.
Show the answer
C. Correct. Gas escapes faster the higher the pressure behind it. Off, the low side holds 1 510 kPa (219 psig), 650 kPa (94 psi) above its 860 kPa (125 psig) running suction, so a slow leak on the coil or suction line puts out the most refrigerant then. The high-side joints are the ones searched with the unit running, when they are at condensing pressure.
Why not the others
A. In cooling the low side runs at its lowest pressure, 860 kPa (125 psig), so the least refrigerant escapes from a leak there. Search those parts with the unit off, when they stand 650 kPa (94 psi) higher.
B. Once the compressor starts, suction pressure falls quickly from the standing value toward its running 860 kPa (125 psig). The low side is at its highest with the unit off and equalized.
D. With the compressor running, suction stays low whether or not the blower runs, and a coil starved of air falls lower still and can ice. Search the low side with the unit off, at its 1 510 kPa (219 psig) standing pressure.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-16.04)
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A 14-year-old R-410A split system with a scroll compressor runs all afternoon without bringing the house down, with superheat and subcooling close to the maker's figures. Against the maker's performance data at the same indoor and outdoor conditions, which pattern points to a compressor that no longer pumps its full capacity?
- Suction pressure low, head pressure low, amps low.
- Suction pressure high, head pressure high, amps high.
- Suction pressure high, head pressure low, amps low.
- Suction pressure low, head pressure normal, amps low.
Show the answer
C. Correct. A compressor with worn scrolls or leaking valves lets gas slip back from the high side to the low side, so suction rises, head falls and the motor does less work. Superheat and subcooling can stay near normal, which is why the pressures and amps against the maker's data tell the story.
Why not the others
A. Low suction and low head with low amps is the shortage pattern, and it comes with high superheat and low subcooling. Here superheat and subcooling are near the maker's figures.
B. Everything high is the pattern of a heavy load, an overcharge or poor condenser airflow. A worn compressor fails to build head pressure, so it cannot give this pattern.
D. Low suction with a normal head and low amps fits a restriction, which also raises superheat. A worn compressor raises suction pressure instead of lowering it.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-14.02)
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An attic air handler's R-410A TXV coil is not keeping up at 31 °C (88 °F) outdoors. Readings: suction 580 kPa (84 psig), dew −4.9 °C; suction line at the coil outlet 11.0 °C; liquid 2 275 kPa (330 psig), bubble 39.2 °C; liquid line 28.5 °C at both the outdoor unit and the TXV inlet, with no drop across the drier. Maker's targets: superheat 5 to 7 K, subcooling 6 K. Which cause fits?
- The system is short of charge.
- The filter-drier is partly plugged.
- Indoor airflow is too low for the coil.
- The TXV is underfeeding the coil.
Show the answer
D. Correct. Superheat 11.0 − (−4.9) = 15.9 K is far above target, yet subcooling of 10.7 K shows the liquid is there and held back. With full-temperature liquid at the valve inlet, the valve itself is not opening enough. Check the bulb's position, clamp and insulation, then the valve's power element.
Why not the others
A. A shortage leaves little liquid to subcool, so subcooling would sit below the 6 K target. Here it is 39.2 − 28.5 = 10.7 K: there is plenty of liquid.
B. A plugged drier drops the liquid temperature across it. The liquid reads 28.5 °C on both sides of the drier and at the TXV inlet, so nothing upstream of the valve restricts it.
C. Low airflow gives the coil less heat to absorb, so superheat falls. Superheat here is 15.9 K, high, so the coil is short of refrigerant, not of air.
Source: STO 313D counselling sheet, skill set D: Service (our sub-task D-14.02)
02Answer key
| Question | Answer | Our task |
|---|---|---|
| Q1 | B | Task D-14 |
| Q2 | A | Task D-15 |
| Q3 | C | Task D-16 |
| Q4 | D | Task D-16 |
| Q5 | C | Task D-16 |
| Q6 | C | Task D-16 |
| Q7 | C | Task D-14 |
| Q8 | D | Task D-14 |
03What skill set D covers
Skill set D is fault-finding and repair on residential systems that have been running. Our four tasks for it are diagnosing; working safely in service and decommissioning equipment; repairing the refrigerant circuit; and repairing air-side and mechanical components. The questions give a set of readings, the unit's history or both, and ask which cause fits all of it: a short charge that was never a leak, a worn compressor, a starving TXV, a compressor killed by poor airflow. Others cover pump-down and recovery limits, unlabelled recovery cylinders, leak searching and the notice O. Reg. 463/10 requires before a unit is scrapped.
STO's 313D exam counselling sheet lists six weighted skill sets and gives Service 21 per cent of the exam, with a pass mark of 70 per cent. The sheet's body says “Updated as of December 5, 2016” and follows the older training standard, not the one in force since December 2023, so its weights may not match today's exam. STO publishes no question count for the 313D exam, only that its exams have between 90 and 150 questions. Our mock is 130 questions in 4 hours, and its draw of 28 questions from skill set D is ours, worked out from the sheet's percentages. The four tasks below and the right-hand column are TicketPrep's own grouping: the column is how many questions our bank holds on each task, Standard and Advanced together, not a count from the exam.
| Our task | In our bank |
|---|---|
| Task D-14 Diagnose | 46 |
| Task D-15 Work safely in service and decommission equipment | 13 |
| Task D-16 Repair the refrigerant circuit | 42 |
| Task D-17 Repair air-side and mechanical components | 23 |
04What the questions turn on
Readings against the maker's data
Superheat, subcooling, pressures and amps each narrow the cause. High superheat with normal or high subcooling, and full-temperature liquid at the valve inlet, means the liquid is there and the TXV is holding it back. A compressor that no longer pumps shows suction high, head low and amps low against the maker's performance data at the same conditions, even when superheat and subcooling look close to normal.
The history often explains the readings
A system that has cooled weakly since day one, holds tight on today's leak test and has only the factory charge on its install sheet was most likely never given its line-set adjustment. A failed scroll with sound windings and clean oil, on a fixed-orifice system with a record of iced coils and a matted filter, points to liquid floodback from low airflow: change the compressor without restoring the airflow and the new one fails the same way.
Pump-down and recovery cylinders have limits
Before pumping a charge into the outdoor unit, compare the whole system charge with the most the unit's manual says it can store, and recover the difference into a cylinder first; overfilling the condenser with liquid leaves no vapour space and the pressure climbs. An unlabelled cylinder is treated as unidentified: R-410A and R-32 sit so close on the pressure-temperature chart at 20 °C (68 °F) that a gauge cannot tell them apart, so a refrigerant identifier settles it.
Leak searches and the end of a unit's life
Gas escapes faster the higher the pressure behind it, so the low side is searched with the system off and the pressures equalized, when it sits well above its running suction, and the high side with the system running. When a unit is retired, O. Reg. 463/10 requires the certified person who finds it holds no refrigerant to affix a notice to that effect, with their ODP certificate details, before it may be dismantled or scrapped.
05The other skill sets
STO publishes no minimum score for any one skill set, only the 70 per cent pass mark on the whole exam, so every skill set's marks count toward it. Each of the six skill sets has its own page of free questions:
The 313D practice exam page describes the full question bank. Working toward the full-scope trade as well? See the 313A practice exam; the two trades share their first-level training. More free questions for every exam we cover: free practice questions. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it. TicketPrep is not affiliated with Skilled Trades Ontario or CSA Group. CSA B52, CSA F280 and the Ontario Electrical Safety Code are cited by name only and never reproduced; Ontario regulations such as O. Reg. 875/21 and O. Reg. 463/10 are quoted only briefly.