EPA 609 Leak Detection & System Diagnosis 2 — Questions and Answers
Question 1: On a properly charged R-134a MVAC system at idle with moderate ambient temperature, typical low-side gauge pressure should be approximately:
- 0–5 psi
- 25–45 psi (Correct answer)
- 70–90 psi
- 110–130 psi
Correct answer: 25–45 psi
A properly functioning R-134a system typically shows 25–45 psi on the low (suction) side at idle, reflecting normal evaporator operating pressure.
Question 2: Abnormally HIGH high-side pressure in an MVAC system most commonly indicates:
- Low refrigerant charge
- A stuck-open expansion valve
- A blocked or inoperative condenser or cooling fan (Correct answer)
- A leaking compressor reed valve
Correct answer: A blocked or inoperative condenser or cooling fan
When the condenser cannot reject heat efficiently due to blockage or fan failure, high-side pressure rises as the refrigerant cannot condense properly.
Question 3: Abnormally LOW high-side pressure combined with abnormally LOW low-side pressure most likely indicates:
- A refrigerant overcharge
- Insufficient refrigerant due to a system leak (Correct answer)
- A faulty pressure cycling switch
- A seized condenser fan
Correct answer: Insufficient refrigerant due to a system leak
Low pressure on both sides is the classic signature of an undercharged system, typically caused by refrigerant loss through a leak.
Question 4: A compressor clutch that rapidly cycles on and off (short-cycles) at higher than normal frequency typically indicates:
- Correct system operation at peak cooling demand
- A low refrigerant charge triggering the low-pressure cutout switch (Correct answer)
- An overcharged system activating the high-pressure relief valve
- A faulty evaporator temperature sensor only
Correct answer: A low refrigerant charge triggering the low-pressure cutout switch
When refrigerant charge is low, suction pressure drops quickly, causing the low-pressure cutout switch to cycle the compressor off repeatedly before pressure can stabilize.
Question 5: A 'bubbling' or 'gurgling' sound heard from the liquid line sight glass (where equipped) indicates:
- The system is properly charged and liquid refrigerant is flowing normally
- Bubbles of refrigerant vapor in the liquid line, suggesting low charge or a restriction (Correct answer)
- Normal refrigerant boiling inside the evaporator
- Excessive oil circulation in the compressor
Correct answer: Bubbles of refrigerant vapor in the liquid line, suggesting low charge or a restriction
Bubbles in the sight glass indicate the liquid line contains refrigerant vapor, which means the charge is insufficient or there is a restriction reducing pressure upstream.
Question 6: High low-side pressure combined with high high-side pressure in an MVAC system can indicate:
- Low refrigerant charge
- A leaking or worn compressor not building adequate pressure differential (Correct answer)
- A blocked orifice tube
- Correct system operation in hot weather
Correct answer: A leaking or worn compressor not building adequate pressure differential
When a compressor cannot pump refrigerant efficiently, pressures equalize at elevated levels — the low side stays high and the high side cannot reach normal peak pressure.
Question 7: After repairing a refrigerant leak, the technician must:
- Immediately recharge the system without further testing
- Perform a vacuum pull-down to remove air and moisture before recharging (Correct answer)
- Add UV dye and charge simultaneously without evacuation
- Pressure test with nitrogen and then skip evacuation
Correct answer: Perform a vacuum pull-down to remove air and moisture before recharging
Evacuating the system to a deep vacuum removes air and moisture contamination introduced during the repair, preventing acid formation and ice blockages after recharging.
EPA 609 Leak Detection & System Diagnosis 2
On a properly charged R-134a MVAC system at idle with moderate ambient temperature, typical low-side gauge pressure should be approximately: