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ASE S7 Air Conditioning Systems & Controls (School Bus) study materials
ASE S7 Air Conditioning Systems & Controls (School Bus) study resources

ASE S7 Air Conditioning Systems & Controls (School Bus) practice questions

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Question 1A/C System Inspection, Diagnosis, and Repair

A school bus A/C system is being inspected and the manifold gauge set reads 28 psi on both the low and high sides with the compressor clutch engaged. Which condition does this most likely indicate?

  1. A blocked condenser airflow
  2. A failed compressor or open internal control valve Correct answer.
  3. A restricted expansion valve
  4. An overcharged system

Explanation

When both gauges read nearly the same low pressure (near the static/low-side value) while the clutch is engaged, the compressor is not building a pressure differential, which points to internal compressor failure or an open control valve. A working system shows a clear high-side/low-side split.

Question 2A/C System Inspection, Diagnosis, and Repair

During an A/C performance test on a school bus, the vent temperature is warm and the low-side gauge reads high while the high side reads low. Which component is the most likely cause?

  1. Restricted condenser
  2. Faulty compressor Correct answer.
  3. Low refrigerant charge
  4. Plugged orifice tube

Explanation

A compressor that cannot pump effectively produces low high-side pressure and elevated low-side pressure, so the system cools poorly. The pressure split is too small because the compressor is not moving and compressing refrigerant properly.

Question 3A/C System Inspection, Diagnosis, and Repair

Which refrigerant is most commonly used in modern school bus A/C systems and operates with a different pressure-temperature relationship than R-134a?

  1. R-12
  2. R-22
  3. R-1234yf Correct answer.
  4. R-410A

Explanation

R-1234yf has largely replaced R-134a in newer vehicles and has a slightly different pressure-temperature curve, so the correct refrigerant must be identified before diagnosing pressures. Using the wrong pressure chart leads to misdiagnosis.

Question 4A/C System Inspection, Diagnosis, and Repair

A technician is recovering refrigerant from a school bus A/C system before repair. Which practice is required to comply with refrigerant handling rules?

  1. Discharge into a sealed container of water
  2. Use certified recovery equipment and capture the refrigerant Correct answer.
  3. Release refrigerant only if the system is below 1 pound
  4. Vent the refrigerant slowly to atmosphere

Explanation

Refrigerant must be recovered with certified equipment rather than vented, because release of refrigerant to the atmosphere is prohibited. Recovery also allows the refrigerant to be recycled or reclaimed.

Question 5A/C System Inspection, Diagnosis, and Repair

An A/C system on a school bus has a strong, musty odor from the vents when the blower runs. Which is the most likely cause?

  1. Microbial growth on the evaporator core Correct answer.
  2. A failing compressor clutch
  3. A restricted liquid line
  4. A refrigerant overcharge

Explanation

Moisture collecting on the evaporator core promotes mold and bacteria growth, producing a musty odor. Cleaning or treating the evaporator and ensuring the drain is clear addresses the source.

Question 6A/C System Inspection, Diagnosis, and Repair

During a visual inspection of a school bus A/C system, oil is found pooling around a line fitting. What does this most likely indicate?

  1. An overcharged system
  2. A refrigerant leak at that connection Correct answer.
  3. Normal condensation drainage
  4. A faulty blower motor

Explanation

Refrigerant carries lubricating oil, so an oily residue at a joint often marks a leak point. The fitting should be leak-checked and repaired.

Question 7A/C System Inspection, Diagnosis, and Repair

A school bus A/C system uses an expansion valve. Which function does the expansion valve perform?

  1. It removes moisture from the refrigerant
  2. It condenses refrigerant into a liquid
  3. It compresses refrigerant vapor
  4. It meters refrigerant and drops pressure into the evaporator Correct answer.

Explanation

The expansion valve throttles liquid refrigerant into the evaporator, dropping pressure so the refrigerant can boil and absorb heat. It also regulates flow based on evaporator outlet temperature or pressure.

Question 8A/C System Inspection, Diagnosis, and Repair

A technician is checking a school bus A/C compressor clutch that will not engage. Which condition should be verified first?

  1. Blower motor speed
  2. Condenser fin condition
  3. Evaporator drain flow
  4. Clutch electrical supply and control signal Correct answer.

Explanation

Before condemning the clutch, verify it is receiving power and a control signal; a no-engage complaint is often electrical (fuse, relay, pressure switch, or control head). Checking the circuit first avoids replacing a good clutch.

Question 9A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a low refrigerant charge. Which gauge reading pattern is typical?

  1. High pressure on both sides
  2. Low pressure on both high and low sides Correct answer.
  3. High low side with low high side
  4. Normal low side with high high side

Explanation

With insufficient refrigerant, both the low-side and high-side pressures read lower than normal because there is less mass to compress and absorb heat. Cooling performance drops accordingly.

Question 10A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a plugged orifice tube. Which symptom is most consistent with this condition?

  1. Equalized pressures with clutch engaged
  2. High low-side pressure and high suction
  3. Very low low-side pressure and a frosted or cold suction line near the restriction Correct answer.
  4. High vent temperature with normal pressures

Explanation

A restriction at the orifice tube starves the evaporator, so low-side pressure falls and the line downstream of the restriction may frost. The high side may read normal or elevated.

Question 11A/C System Inspection, Diagnosis, and Repair

During an A/C inspection, a technician notices the condenser is packed with road debris and mud. What effect does this have on system operation?

  1. Low high-side pressure and icing evaporator
  2. High high-side pressure and reduced cooling Correct answer.
  3. Normal operation with lower amp draw
  4. Low low-side pressure only

Explanation

A condenser that cannot reject heat raises high-side pressure and reduces cooling capacity. Cleaning the condenser restores heat transfer and normal pressures.

Question 12A/C System Inspection, Diagnosis, and Repair

A school bus A/C system uses an accumulator. Where is the accumulator located in the refrigerant circuit?

  1. Between the condenser and the expansion device
  2. Between the evaporator and the compressor Correct answer.
  3. Inside the evaporator core
  4. Between the compressor and the condenser

Explanation

On an orifice-tube system, the accumulator sits on the low side between the evaporator outlet and the compressor inlet, storing liquid and holding desiccant to protect the compressor from slugging.

Question 13A/C System Inspection, Diagnosis, and Repair

A technician is performing an A/C performance test on a school bus. Which set of conditions best supports a valid comparison to specifications?

  1. Engine at idle, doors open, blower on low
  2. Engine off, key on, blower on high
  3. Engine at idle with recirculation off and windows open
  4. Engine at specified RPM, doors closed, blower on high, max cool Correct answer.

Explanation

Performance testing requires stable, repeatable conditions: specified engine speed, maximum blower, max cool, and doors/windows closed so the cabin load is controlled. Deviating from these conditions makes vent temperature meaningless.

Question 14A/C System Inspection, Diagnosis, and Repair

An A/C system on a school bus cools poorly at idle but cools well at highway speed. Which condition best explains this?

  1. Insufficient condenser airflow at low speed Correct answer.
  2. A plugged expansion valve
  3. An overcharged system
  4. A weak compressor

Explanation

If cooling improves as vehicle speed and airflow increase, condenser airflow at idle is likely inadequate (fan clutch, electric fan, or debris). Higher airflow at speed restores heat rejection.

Question 15A/C System Inspection, Diagnosis, and Repair

A technician finds the A/C suction line sweating and cold but the vent air is only mildly cool. Which is the most likely explanation?

  1. The blend door is allowing heated air to mix with cooled air Correct answer.
  2. The evaporator is flooded with liquid refrigerant
  3. The condenser is over-cooling
  4. The compressor clutch is slipping

Explanation

A cold, sweating suction line shows the evaporator is refrigerating, so poor vent temperature often comes from air-side problems such as a mispositioned blend door mixing hot air. The refrigeration circuit can be working while delivery is compromised.

Question 16A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a leak that cannot be found with a refrigerant detector or dye. Which method is appropriate for locating a small, slow leak?

  1. Overcharge the system and look for oil
  2. Run the system with the clutch disabled
  3. Pressurize with nitrogen and use an ultrasonic or soap-bubble test Correct answer.
  4. Vent refrigerant and watch for frost

Explanation

Nitrogen pressure testing with an ultrasonic detector or soap solution can reveal small leaks without adding refrigerant. It is a standard approach when electronic leak detection is inconclusive.

Question 17A/C System Inspection, Diagnosis, and Repair

A technician is diagnosing an A/C system with an expansion valve and a receiver-drier. The sight glass shows continuous bubbles. What does this most likely indicate?

  1. A fully charged system
  2. A blocked evaporator drain
  3. A possible low charge or vapor in the liquid line Correct answer.
  4. A failed compressor

Explanation

Continuous bubbles at the sight glass suggest the liquid line is not solid liquid, often from a low charge or air in the system. The charge should be verified by weight or pressure before adding refrigerant.

Question 18A/C System Inspection, Diagnosis, and Repair

A school bus A/C system uses an expansion valve. The evaporator is freezing and the low-side pressure is very low. Which component is the most likely cause?

  1. A failed blower motor resistor
  2. A stuck-closed expansion valve Correct answer.
  3. A restricted condenser
  4. An overcharged system

Explanation

An expansion valve stuck closed starves the evaporator, causing very low low-side pressure and evaporator freeze-up. The valve should be tested and replaced if it cannot regulate flow.

Question 19A/C System Inspection, Diagnosis, and Repair

Which safety precaution is most important when servicing the A/C system on a school bus?

  1. Disconnect the battery only after opening the system
  2. Loosen fittings quickly to speed recovery
  3. Use compressed air to pressure-test the system
  4. Wear eye protection and avoid contact with liquid refrigerant Correct answer.

Explanation

Liquid refrigerant causes frostbite and eye injury, so eye protection and gloves are essential. Fittings should never be opened while the system is pressurized.

Question 20A/C System Inspection, Diagnosis, and Repair

A school bus A/C system's compressor is noisy and the high-side pressure is low while the low side is high. What is the most likely internal fault?

  1. A worn compressor with leaking reed valves Correct answer.
  2. A faulty blower motor
  3. A restricted liquid line
  4. A clogged evaporator drain

Explanation

Worn compressor reed valves prevent effective compression, producing a small pressure split and noise. The compressor should be tested and replaced if it cannot build differential pressure.

Question 21A/C System Inspection, Diagnosis, and Repair

A technician is checking the A/C electrical controls on a school bus. The compressor clutch engages only when the pressure switch is bypassed. What does this indicate?

  1. The pressure switch circuit is open or the switch has failed Correct answer.
  2. The condenser is restricted
  3. The clutch coil is open
  4. The compressor is mechanically seized

Explanation

If bypassing the pressure switch restores clutch engagement, the switch or its circuit is interrupting the control signal. The switch should be tested for continuity and proper pressure response.

Question 22A/C System Inspection, Diagnosis, and Repair

During an A/C inspection, a technician finds the evaporator drain tube is clogged. Which symptom is most likely?

  1. Compressor clutch slipping
  2. Low refrigerant charge
  3. Water accumulating in the cabin or on the floor Correct answer.
  4. High high-side pressure

Explanation

A clogged evaporator drain traps condensate, which can overflow into the cabin and cause odors or water damage. Clearing the drain restores proper drainage.

Question 23A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a blocked expansion valve screen. Which reading is most likely?

  1. Both gauges reading zero with the engine running
  2. Low low-side pressure with a normal high side Correct answer.
  3. High low-side pressure with a low high side
  4. Equalized pressures with the clutch engaged

Explanation

A blocked screen restricts refrigerant flow to the evaporator, dropping low-side pressure while the high side stays near normal or rises slightly. The screen should be cleaned or replaced.

Question 24A/C System Inspection, Diagnosis, and Repair

Which procedure correctly checks for non-condensable gases (air) in a school bus A/C system?

  1. Compare high-side pressure to the pressure-temperature chart for the ambient temperature Correct answer.
  2. Measure vent temperature only
  3. Check the compressor clutch amp draw
  4. Watch the blower motor speed

Explanation

Air in the system raises high-side pressure above the saturation pressure for the measured ambient temperature. Comparing gauge readings to the refrigerant's pressure-temperature chart identifies non-condensables.

Question 25A/C System Inspection, Diagnosis, and Repair

A technician must add refrigerant to a school bus A/C system. Which method is preferred to ensure the correct charge?

  1. Add refrigerant until the sight glass is clear
  2. Add refrigerant until the high side reaches 300 psi
  3. Estimate the charge by vent temperature
  4. Charge by weight using a recovery/recycling machine Correct answer.

Explanation

Charging by weight is the most accurate method and is specified by manufacturers. Gauge or sight-glass methods are unreliable and can lead to under- or overcharge.

Question 26A/C System Inspection, Diagnosis, and Repair

Which component in a school bus A/C system is responsible for removing moisture from the refrigerant?

  1. Evaporator
  2. Condenser
  3. Receiver-drier Correct answer.
  4. Expansion valve

Explanation

The receiver-drier contains a desiccant that absorbs moisture from the refrigerant. The expansion valve meters refrigerant, the condenser releases heat, and the evaporator absorbs heat.

Question 27A/C System Inspection, Diagnosis, and Repair

A school bus A/C system is being tested with a manifold gauge set. The low-side pressure is 45 psi and the high-side pressure is 425 psi. What is the most likely cause?

  1. Faulty expansion valve
  2. Restricted condenser airflow Correct answer.
  3. Weak compressor
  4. Low refrigerant charge

Explanation

High high-side pressure with normal or elevated low-side pressure often indicates poor heat rejection at the condenser. Restricted airflow reduces the condenser's ability to release heat, raising high-side pressure.

Question 28A/C System Inspection, Diagnosis, and Repair

When replacing a failed A/C compressor on a school bus, which practice is recommended to protect the new compressor?

  1. Bypass the pressure switch
  2. Reuse the old receiver-drier to save cost
  3. Flush the system and replace the receiver-drier Correct answer.
  4. Add extra refrigerant oil to the system

Explanation

Flushing removes debris from a failed compressor, and replacing the receiver-drier ensures moisture and contaminants are captured. Reusing the old drier risks releasing debris and moisture into the new compressor.

Question 29A/C System Inspection, Diagnosis, and Repair

A school bus A/C system uses an expansion valve. The technician observes frost on the suction line right after the evaporator. Which condition is most likely?

  1. Overcharged system
  2. Failed blower motor
  3. Restricted expansion valve Correct answer.
  4. Normal operation

Explanation

Frost on the suction line indicates that refrigerant is evaporating too early, often due to a restriction at the expansion valve. This causes low pressure and temperature downstream of the restriction.

Question 30A/C System Inspection, Diagnosis, and Repair

Which tool is best for detecting a small refrigerant leak in a school bus A/C system?

  1. Electronic refrigerant leak detector Correct answer.
  2. Infrared thermometer
  3. Manifold gauge set
  4. Soap bubble solution

Explanation

An electronic refrigerant leak detector is sensitive to small concentrations of refrigerant and is effective for pinpointing small leaks. Soap bubbles are useful for larger leaks, and thermometers and gauges do not detect leaks.

Question 31A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a suction line that is cold and sweating, but the vent air is not cool. Which component is most likely faulty?

  1. Blower motor Correct answer.
  2. Expansion valve
  3. Condenser fan
  4. Compressor clutch

Explanation

A cold, sweating suction line indicates the evaporator is receiving refrigerant and is cold. If the blower motor is not moving air across the evaporator, the cold remains in the evaporator and does not reach the vents.

Question 32A/C System Inspection, Diagnosis, and Repair

What is the correct procedure for checking the refrigerant charge in a school bus A/C system using the sight glass?

  1. Check for a clear sight glass with the engine off
  2. Check for a clear sight glass with the system running Correct answer.
  3. Check for bubbles with the A/C off
  4. Check for bubbles with the engine off

Explanation

With the system running and stabilized, a clear sight glass (no bubbles) generally indicates a full charge. Bubbles indicate a low charge or air in the system.

Question 33A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a low refrigerant charge. Which symptom is most likely to be observed?

  1. High high-side pressure
  2. Normal vent temperatures
  3. Low low-side pressure Correct answer.
  4. Frequent compressor clutch cycling

Explanation

A low charge results in reduced refrigerant flow, causing low pressure on both the low and high sides. The low-side pressure will be noticeably lower than normal.

Question 34A/C System Inspection, Diagnosis, and Repair

A technician is diagnosing an A/C system with an expansion valve. The low-side pressure is normal, but the high-side pressure is low. What is the most likely cause?

  1. Weak compressor Correct answer.
  2. Faulty expansion valve
  3. Restricted condenser
  4. Overcharged system

Explanation

A weak compressor cannot build normal high-side pressure, resulting in low high-side readings while the low side may appear normal. Other symptoms include poor cooling.

Question 35A/C System Inspection, Diagnosis, and Repair

Which safety practice is essential when working on a school bus A/C system?

  1. Wear safety glasses and gloves
  2. Work in a well-ventilated area
  3. All of the above
  4. Recover refrigerant before opening the system Correct answer.

Explanation

Refrigerant must be recovered to prevent release into the atmosphere, which is both an environmental and safety hazard. While PPE and ventilation are important, recovery is a specific and critical step.

Question 36A/C System Inspection, Diagnosis, and Repair

A school bus A/C system has a blocked expansion valve screen. Which gauge reading is most likely?

  1. Normal low-side pressure and high high-side pressure
  2. Low low-side pressure and low high-side pressure Correct answer.
  3. Low low-side pressure and normal high-side pressure
  4. High low-side pressure and high high-side pressure

Explanation

A blocked screen restricts refrigerant flow, causing low pressure on the low side and reduced refrigerant entering the evaporator, which leads to low high-side pressure as well.

Question 37A/C System Inspection, Diagnosis, and Repair

What is the preferred method to add refrigerant to a school bus A/C system to ensure the correct charge?

  1. Add refrigerant until the sight glass is clear
  2. Charge until the low-side pressure reaches 40 psi
  3. Charge by weight using a refrigerant scale Correct answer.
  4. Charge until the vents blow cold

Explanation

Charging by weight is the most accurate method because it measures the exact amount of refrigerant added. Other methods are approximate and can lead to under- or overcharge.

Question 38A/C System Inspection, Diagnosis, and Repair

A school bus A/C system uses a thermal expansion valve. The evaporator is freezing and the low-side pressure is very low. Which component is most likely faulty?

  1. Compressor
  2. Expansion valve Correct answer.
  3. Condenser fan
  4. Receiver-drier

Explanation

A faulty expansion valve stuck closed or restricted will cause very low low-side pressure and allow the evaporator to freeze due to insufficient refrigerant flow.

Question 39A/C System Inspection, Diagnosis, and Repair

Which component in a school bus A/C system is responsible for engaging the compressor when cooling is requested?

  1. Pressure switch
  2. Compressor clutch Correct answer.
  3. Thermostatic switch
  4. Blower motor resistor

Explanation

The compressor clutch is an electromagnetic clutch that engages the compressor when powered. It is controlled by the A/C control head and various switches.

Question 40A/C System Inspection, Diagnosis, and Repair

A technician is inspecting a school bus A/C system and finds the condenser fins are bent and clogged with debris. What effect does this have on system performance?

  1. Reduced cooling efficiency Correct answer.
  2. Lower compressor load
  3. Reduced high-side pressure
  4. Increased cooling capacity

Explanation

A clogged condenser cannot release heat effectively, leading to higher high-side pressure and reduced cooling efficiency. The system must work harder to reject heat.

Question 41Refrigerant Recovery, Recycling, and Handling

Under EPA regulations, refrigerant that is recovered from a school bus air conditioning system during service must be handled in what way?

  1. Vented to the atmosphere if the quantity is small
  2. Burned in an approved shop heater
  3. Recycled or reclaimed before it can be charged back into a vehicle Correct answer.
  4. Discharged into the shop drain with water

Explanation

Refrigerant removed during service must be recovered and then recycled or reclaimed before reuse; venting ozone-depleting refrigerant is prohibited. Small quantities are not exempt from the recovery requirement.

Question 42Refrigerant Recovery, Recycling, and Handling

A recovery machine is rated by its ability to pull a system down to a specified vacuum. What does this rating describe?

  1. Oil return rate
  2. Leak rate
  3. Recovery efficiency Correct answer.
  4. Charge tolerance

Explanation

Recovery machines are rated by how completely they can evacuate refrigerant from a system, which is expressed as recovery efficiency. This rating indicates the percentage of refrigerant removed under specified conditions.

Question 43Refrigerant Recovery, Recycling, and Handling

When recovering refrigerant from a system, which practice helps prevent contamination of the recovered refrigerant?

  1. Recovering mixed refrigerants into a single tank
  2. Using dedicated service equipment for each refrigerant type Correct answer.
  3. Pulling a deep vacuum before connecting hoses
  4. Adding fresh oil to the recovery tank

Explanation

Dedicated equipment for each refrigerant prevents cross-contamination so the recovered refrigerant can be recycled or reclaimed. Mixing refrigerants creates an unidentifiable blend that cannot be properly processed.

Question 44Refrigerant Recovery, Recycling, and Handling

What is the primary purpose of a refrigerant identifier used before recovery?

  1. To measure system operating pressures
  2. To test the compressor clutch circuit
  3. To check the refrigerant charge weight
  4. To determine the type and purity of refrigerant in the system Correct answer.

Explanation

An identifier analyzes the refrigerant to confirm its type and detect contamination or mixed blends before recovery. This prevents unknown refrigerant from entering a recycling machine or storage cylinder.

Question 45Refrigerant Recovery, Recycling, and Handling

Refrigerant storage cylinders should never be filled beyond what limit?

  1. 50 percent of the cylinder capacity by weight
  2. 80 percent of the cylinder capacity by weight Correct answer.
  3. 100 percent of the cylinder capacity by weight
  4. 95 percent of the cylinder capacity by weight

Explanation

Cylinders are limited to about 80 percent of capacity to leave room for liquid expansion as temperature rises. Overfilling can lead to hydraulic pressure buildup and cylinder rupture.

Question 46Refrigerant Recovery, Recycling, and Handling

Which personal protective equipment is most important when handling refrigerant during recovery?

  1. Hearing protection
  2. Respirator with dust filters
  3. Safety goggles and gloves Correct answer.
  4. Steel-toed boots

Explanation

Liquid refrigerant can cause frostbite and eye injury, so safety goggles and appropriate gloves are essential. Refrigerant contact with skin or eyes is the primary hazard during recovery work.

Question 47Refrigerant Recovery, Recycling, and Handling

A technician recovers refrigerant into a cylinder that is warm to the touch. What effect does a warm recovery cylinder have on the recovery process?

  1. It has no effect on recovery speed
  2. It raises internal pressure and slows recovery Correct answer.
  3. It causes the recovery machine to shut off automatically
  4. It lowers internal pressure and speeds recovery

Explanation

A warm cylinder has higher internal pressure, which reduces the pressure differential driving refrigerant into it and slows recovery. Keeping the cylinder cool improves recovery speed.

Question 48Refrigerant Recovery, Recycling, and Handling

After recovering refrigerant from a system, a technician notices the recovery cylinder is heavier than expected for the amount removed. What is the most likely concern?

  1. The scale is always inaccurate after recovery
  2. The refrigerant has turned into a solid
  3. The cylinder may be overfilled and should be checked against its capacity Correct answer.
  4. The recovery machine added extra refrigerant

Explanation

An unexpectedly heavy cylinder may indicate overfilling, which risks overpressure and rupture. The technician should verify the weight against the cylinder's 80 percent limit.

Question 49Refrigerant Recovery, Recycling, and Handling

Why must recovered refrigerant be recycled through a filter-drier before it is reused in the same vehicle?

  1. To add lubricant back into the refrigerant
  2. To increase the refrigerant's boiling point
  3. To change the refrigerant into a different type
  4. To remove moisture and particulate contamination Correct answer.

Explanation

Recycling passes refrigerant through filter-driers that capture moisture, acid, and particulates so it meets reuse standards. Removing contaminants protects the system from corrosion and blockage.

Question 50Refrigerant Recovery, Recycling, and Handling

A recovery machine's filter-drier becomes saturated. What symptom is most likely during recovery?

  1. The recovery cylinder cools rapidly
  2. The refrigerant changes color to green
  3. The manifold gauge set reads zero pressure
  4. Recovery slows and the machine may not reach the required vacuum Correct answer.

Explanation

A saturated filter-drier restricts flow, slowing recovery and preventing the machine from reaching its rated vacuum. Replacing the filter-drier restores proper performance.