CRST Troubleshooting & Repair Techniques 3 — Questions and Answers
Question 1: A TXV-equipped system has low suction pressure and low superheat simultaneously. What is the MOST likely cause?
- TXV stuck closed (Correct answer)
- TXV stuck open
- Refrigerant undercharge
- Liquid line restriction
Correct answer: TXV stuck closed
A TXV stuck in the closed position starves the evaporator, causing low suction pressure; low superheat indicates little refrigerant is vaporizing past the valve.
Question 2: What does a bubbling or foamy sight glass indicate in a refrigeration system?
- Normal subcooling
- Refrigerant overcharge
- Moisture in the system
- Low refrigerant or liquid line restriction (Correct answer)
Correct answer: Low refrigerant or liquid line restriction
Bubbles in the sight glass indicate the liquid refrigerant is flashing to vapor, which points to a low charge or restriction upstream.
Question 3: A compressor that hums but fails to start most likely has a faulty:
- Discharge valve
- Start capacitor or start relay (Correct answer)
- Crankcase heater
- Low-pressure switch
Correct answer: Start capacitor or start relay
A failed start capacitor or start relay prevents the start winding from providing enough torque to overcome compressor inertia during startup.
Question 4: Frost forming on the suction line all the way back to the compressor indicates:
- Low superheat and possible liquid floodback (Correct answer)
- High superheat
- Refrigerant undercharge
- Condenser fan failure
Correct answer: Low superheat and possible liquid floodback
Suction line frost extending to the compressor indicates liquid refrigerant is returning, which can cause liquid slugging and compressor damage.
Question 5: When recovering refrigerant from a system with a hermetic compressor burnout, the technician should:
- Recover normally and reuse the refrigerant
- Use a dedicated recovery cylinder and filter to avoid cross-contamination (Correct answer)
- Vent the refrigerant outdoors
- Only recover if the refrigerant is a blend
Correct answer: Use a dedicated recovery cylinder and filter to avoid cross-contamination
Burnout contaminates refrigerant with acids and carbon; it must be recovered into a dedicated cylinder and sent for reclamation or disposal.
Question 6: A refrigeration system loses capacity gradually over several weeks. The most cost-effective FIRST diagnostic step is to:
- Replace the compressor
- Check and record system operating pressures and temperatures (Correct answer)
- Add refrigerant immediately
- Replace the filter-drier
Correct answer: Check and record system operating pressures and temperatures
Measuring and recording operating pressures and temperatures provides a baseline to compare against design specs and identify the root cause systematically.
Question 7: High condensing pressure accompanied by a hot condenser inlet and a cool condenser outlet MOST likely indicates:
- Non-condensables in the system
- Refrigerant overcharge
- Condenser coil fouled with dirt (Correct answer)
- Liquid line solenoid stuck closed
Correct answer: Condenser coil fouled with dirt
A fouled condenser coil reduces heat transfer, causing high condensing pressure; the coil feels hot at the inlet but cool at the outlet because heat is not transferring through the dirty coil.
CRST Troubleshooting & Repair Techniques 3
A TXV-equipped system has low suction pressure and low superheat simultaneously.
What is the MOST likely cause?