313D Troubleshooting and System Diagnosis 2 — Questions and Answers
Question 1: A technician measures suction pressure significantly above normal and discharge pressure below normal on a residential split system. What is the most likely cause?
- Refrigerant overcharge
- Compressor valve failure (Correct answer)
- Restricted liquid line filter-drier
- Low refrigerant charge
Correct answer: Compressor valve failure
Compressor valve failure causes high suction and low discharge pressure because refrigerant bypasses the compressor valves instead of being properly compressed.
Question 2: During a service call, a homeowner reports the system runs continuously but never reaches setpoint. The evaporator coil is partially frosted. What should the technician check first?
- Thermostat calibration
- Condenser fan motor
- Airflow across the evaporator coil (Correct answer)
- Compressor amperage draw
Correct answer: Airflow across the evaporator coil
Restricted airflow is the most common cause of evaporator coil frosting and inadequate cooling, and should be checked before electrical components.
Question 3: A technician finds the high-pressure switch tripping repeatedly on a hot day. Condenser coil appears clean. What is the next most likely cause?
- Low refrigerant charge
- Condenser fan motor failure or reduced airflow (Correct answer)
- Metering device restriction
- Compressor short cycling
Correct answer: Condenser fan motor failure or reduced airflow
A faulty or undersized condenser fan results in inadequate heat rejection, causing head pressure to rise and trip the high-pressure switch.
Question 4: What does a high superheat AND high subcooling reading together most likely indicate?
- Refrigerant overcharge
- Refrigerant undercharge
- Restriction in the liquid line or metering device (Correct answer)
- Compressor valve failure
Correct answer: Restriction in the liquid line or metering device
High subcooling indicates liquid refrigerant is backing up before a restriction, while high superheat indicates insufficient refrigerant reaching the evaporator.
Question 5: A technician suspects a refrigerant leak. After recovering and recharging, what is the correct procedure before returning the system to service?
- Pressurize with nitrogen and perform a leak test (Correct answer)
- Immediately evacuate to 500 microns and recharge
- Run the system and monitor pressures for 30 minutes
- Add UV dye and charge with refrigerant to find the leak location
Correct answer: Pressurize with nitrogen and perform a leak test
After any repair, the system must be pressure-tested with nitrogen to confirm the repair is leak-free before evacuation and recharge.
Question 6: A residential system has correct refrigerant charge but the supply air temperature differential (delta-T) is only 10°F instead of the expected 16–22°F. What is the most likely cause?
- Overcharge of refrigerant
- Excessive airflow across the evaporator (Correct answer)
- Refrigerant undercharge
- Liquid line restriction
Correct answer: Excessive airflow across the evaporator
Too much airflow across the evaporator reduces the time the air has to exchange heat, resulting in a lower temperature differential.
Question 7: When using an electronic leak detector and no leak is found at suspected areas, what technique should the technician use next?
- Recover all refrigerant and pressure test with nitrogen
- Increase system pressure by adding refrigerant
- Use UV dye and a UV light to locate the leak (Correct answer)
- Replace the entire refrigerant circuit
Correct answer: Use UV dye and a UV light to locate the leak
UV dye inspection is an effective secondary method to locate leaks that electronic detectors may miss, especially slow or intermittent leaks.
313D Troubleshooting and System Diagnosis 2
A technician measures suction pressure significantly above normal and discharge pressure below normal on a residential split system.
What is the most likely cause?