Your FREE ASE A7 Flashcards 2026 – 200+ Cards

Heating, Ventilation, and Air Conditioning — the vehicle system that heats, cools, and circulates air in the passenger compartment.

The closed loop in which refrigerant absorbs cabin heat at the evaporator, is compressed, releases that heat at the condenser, and is metered back to low pressure.

Refrigerant boils in the evaporator absorbing cabin heat, the compressor raises its pressure, the condenser dumps that heat outside, and the metering device drops it back to low pressure to repeat.

The high-pressure, hot portion of the A/C system — from the compressor outlet through the condenser to the metering device.

The low-pressure, cold portion of the A/C system — from the metering device through the evaporator back to the compressor inlet.

The metering device (TXV or orifice tube), just before the evaporator.

The service tool that connects to the high and low sides to read system pressures during diagnosis, recovery, and recharge.

A low refrigerant charge — the system is undercharged, usually from a leak.

An overcharge, air (non-condensables) in the system, a dead condenser fan, or a blocked condenser.

A weak or failed compressor that is not pumping properly.

A restriction — a clogged orifice tube, a stuck-closed TXV, or moisture freezing at the metering device.

The condenser cannot reject heat — most often a dead cooling/condenser fan or a debris-blocked condenser.

Blocked/dirty condenser, inoperative condenser fan, overcharge, air in the system, or a high-side restriction.

Cooling refrigerant liquid below its condensing temperature on the high side; proper subcooling indicates a correctly charged condenser.

Heating refrigerant vapor above its boiling temperature on the low side; the TXV controls superheat to keep liquid out of the compressor.

Cabin heat comes from engine coolant through the heater core, so A7 tests the radiator, water pump, thermostat, fans, and coolant.

A small radiator in the dash; engine coolant flows through it and cabin air blown across it provides heat — the heating half of HVAC.

A coolant-side problem: low coolant level, air in the system, a thermostat stuck open, or a partially plugged heater core.

A plugged heater core or a stuck blend door — the coolant is hot but heat isn't reaching the cabin.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms, then opens to regulate operating temperature.

Causes engine overheating — coolant cannot flow to the radiator.

Keeps the engine running too cool — poor fuel economy and weak cabin heat.

The belt- or chain-driven pump that circulates coolant through the engine, radiator, and heater core.

The engine-cooling heat exchanger where airflow sheds combustion heat from the coolant; its pressure cap raises the coolant boiling point.

Seals the cooling system and holds pressure, raising the coolant boiling point so the system can run hotter without boiling.

Pulls air through the radiator and the condenser; with A/C on it should run to keep condenser (high-side) pressure down, especially at idle.

Trapped air pockets block coolant flow, causing weak heat and uneven temperatures; the system must be bled/burped after refill.

A head-gasket leak — bubbles in the overflow or a positive block tester indicate combustion gas entering the cooling system.

An ethylene- or propylene-glycol mix with water that lowers freeze point, raises boil point, and protects against corrosion.

Tools that check coolant freeze protection and concentration; a refractometer is the more accurate.

Fresh mode pulls outside air in; recirculate reuses cabin air for faster cooling and to keep out odors/dust.

The evaporator dehumidifies the air; condensed moisture drains out through the evaporator case drain — normal operation.

Too-low evaporator temperature (or a stuck door) freezes condensate on the coil, blocking airflow and killing cooling until it thaws.

Comparing measured vent (duct) temperature and system pressures against spec, at a set RPM and conditions, to judge cooling performance.

Airflow from vehicle speed that helps the condenser reject heat; its loss at idle is why the cooling fan matters most then.

A thermostatic or viscous clutch that engages a belt-driven fan when the engine is hot; failure causes idle/low-speed overheating.

A motor-driven fan commanded by the PCM/relay based on coolant temp and A/C demand; failure raises temps at idle.

The single belt that drives the A/C compressor, water pump, alternator, and accessories; a slipping or broken belt stops the compressor and pump.

Applying pressure with a hand pump and gauge to find external/internal coolant leaks and to test the radiator cap.

Possible low coolant, stuck-closed thermostat, failed water pump, dead cooling fan, or a head-gasket leak.

On some vehicles, a valve that controls coolant flow to the heater core for temperature control; a stuck valve causes no-heat or always-hot.

Airflow split to both the dash (panel) and floor vents at once.

Directs airflow to the windshield; usually runs the A/C compressor to dehumidify even when set warm, clearing fog.

The evaporator removes moisture from the air, so dehumidified air clears and prevents windshield fogging faster.

Possible undercharge, restriction, blend/door fault routing air past the evaporator, or a TXV/orifice problem.

Mold/mildew on the damp evaporator; clean/treat the evaporator and replace the cabin air filter.

Often a restricted radiator or low coolant flow (weak water pump); at low speed the fan compensates.

Usually a cooling-fan or fan-clutch problem — no ram air, so the fan must move the air.

Stores coolant pushed out as it expands when hot and returns it as the engine cools through the cap's vacuum valve.

Removing trapped air after a refill (bleeder valves, fill funnel, or vacuum fill) so coolant circulates and heat returns.

Coolant and oil are mixing — a blown head gasket, cracked head/block, or failed oil cooler.

Coolant burning in the combustion chamber — a head-gasket or cooling-system internal leak.

A fluorescent dye added to the A/C (or coolant) glows under UV light at the leak point for hard-to-find leaks.

A handheld tool that detects escaping refrigerant near fittings and components to pinpoint A/C leaks.

Pressurizing an open or evacuated A/C system with dry nitrogen to find leaks without venting refrigerant.

For a given refrigerant, pressure corresponds to temperature; gauges and a P-T chart let you infer the refrigerant's state.

A properly performing A/C system typically delivers vent air far below ambient (often roughly 40-50°F / ~5-10°C at the center vent).

Selects fresh outside air or recirculated cabin air at the blower inlet; recirculate speeds cooling on hot days.

Reports interior temperature to the climate-control module; often aspirated by a small fan for an accurate reading.

Trapped air (an air-bound system) — the system needs to be bled so coolant flows through the heater core.

A plugged case drain lets condensate pool, causing wet floors, fogging, and odor; clear the drain tube.

Rapid clutch on/off from a low charge or a faulty cycling switch; cooling is weak and pressures swing.

Keeps correct tension on the serpentine belt that drives the compressor and water pump; a weak tensioner causes slip, squeal, and weak A/C/cooling.

Use the specified coolant (IAT, OAT, HOAT); mixing incompatible types can gel or reduce corrosion protection.

A fluid that changes color when exhaust gas is present in the coolant, confirming a head-gasket/internal leak.

Scale or debris blocks tubes, causing overheating especially at speed; an infrared scan shows cold spots on the core.

It carries high-pressure, high-temperature refrigerant from the compressor through the condenser — normal until it condenses.

Low-pressure refrigerant boils in the evaporator, absorbing heat, so the suction line returns cold to the compressor.

Sweet smell, foggy windshield film, damp passenger floor, and unexplained coolant loss.

Designs that separate air from coolant to prevent air pockets and hot spots; helps keep heat consistent.

Reverse-flushing a partially plugged heater core can restore heat; a fully blocked or leaking core is replaced.

Air flows through the condenser first, then the radiator; a dirty condenser also raises engine coolant temps.

The working fluid that carries heat through the A/C system by changing between liquid and vapor as pressure changes.

An older CFC refrigerant (Freon) phased out because it depletes the ozone layer; systems were retrofitted to R-134a.

A hydrofluorocarbon refrigerant that replaced R-12; the standard automotive refrigerant for decades. A greenhouse gas — must be recovered, not vented.

A low-global-warming-potential refrigerant on newer vehicles in place of R-134a; mildly flammable, with different fittings, oil, and equipment.

No — R-12, R-134a, and R-1234yf use different fittings, oils, and procedures and must never be mixed; identify the type from the underhood label.

The belt-driven pump of the A/C system that compresses low-pressure vapor into high-pressure, high-temperature vapor and circulates refrigerant.

A heat exchanger in front of the radiator where the hot high-pressure vapor releases heat to outside air and condenses into a high-pressure liquid.

The cooling coil inside the dash where low-pressure liquid refrigerant boils into vapor, absorbing cabin heat and dehumidifying the air.

The component (TXV or orifice tube) that restricts refrigerant flow and divides the high side from the low side just before the evaporator.

A metering device that varies refrigerant flow into the evaporator based on temperature and pressure; paired with a high-side receiver-drier.

A fixed-restriction metering device that drops high-pressure liquid to low pressure; paired with a low-side accumulator.

A high-side canister in TXV systems that stores refrigerant, filters debris, and uses desiccant to remove moisture.

A low-side canister in orifice-tube systems that stores excess refrigerant, removes moisture, and keeps liquid out of the compressor.

TXV systems use a high-side receiver-drier; orifice-tube systems use a low-side accumulator — a classic A7 distinction.

Absorbs moisture from the refrigerant to prevent acid formation and ice/freeze-up at the metering device.

Its desiccant becomes saturated once exposed to air/moisture; a fresh drier protects the rebuilt system.

Starves the evaporator — a near-vacuum low side, poor cooling, and frost at the restriction.

Restricts flow into the evaporator, pulling the low side toward a vacuum and causing weak cooling.

Overfeeds the evaporator, flooding it and possibly returning liquid to the compressor (slugging).

An electromagnetic coupling that engages the compressor to the drive pulley when cooling is demanded and disengages when A/C is off or a switch opens.

A compressor that varies its output to match demand (often clutchless), holding evaporator temperature without rapid cycling.

Polyalkylene glycol oil used to lubricate R-134a/R-1234yf compressors; must match the system's specified viscosity and refrigerant.

Polyol ester oil used in some A/C systems (and many R-1234yf/electric-compressor applications); use only the oil the maker specifies.

Poor lubrication and possible compressor failure or system damage; always use the specified oil type and amount.

Capturing refrigerant from a system with approved equipment instead of venting it, as required by Clean Air Act Section 609.

Cleaning recovered refrigerant — removing moisture, oil, and debris — so it meets purity standards for reuse.

Using a vacuum pump to pull the system into a deep vacuum after recovery, boiling off and removing air and moisture before recharging.

A deep vacuum boils off moisture and removes air; moisture forms acids and freezes at the metering device.

A/C systems are charged by weight (from a scale), not by feel; over- or undercharging both hurt cooling.

Both pressures high and weak cooling; recover, evacuate, and recharge to the exact specified weight.

Both pressures low, weak/intermittent cooling, and clutch cycling — usually from a refrigerant leak.

A tool that confirms the refrigerant type and purity before service so contaminated refrigerant isn't pulled into recovery equipment.

Air trapped in the A/C system raises high-side pressure and hurts cooling; removed by proper recovery and deep evacuation.

Knocking or rattling can mean internal wear, liquid slugging, or a failing clutch bearing; debris from a failed compressor can spread system-wide.

Severe internal compressor breakdown that contaminates the system with debris and acid, often requiring a full flush and component replacement.

Cleaning out oil and debris (e.g., after compressor failure) with approved solvent/equipment before installing new parts.

Bugs, dirt, or bent fins block airflow, raising high-side pressure and reducing cooling — clean and straighten the fins.

An efficient condenser design; if internally restricted it can't be flushed and is typically replaced.

The high- and low-side fittings used to connect gauges and equipment; R-134a, R-12, and R-1234yf use different, non-interchangeable fittings.

A spring-loaded check valve in the service port that seals the system; a leaking one can cause slow refrigerant loss.

The state where liquid and vapor coexist; pressure and temperature track each other, which is how P-T charts work.

The high-side line carrying high-pressure liquid refrigerant from the condenser to the metering device.

The low-side line carrying low-pressure vapor from the evaporator back to the compressor; the larger, cooler line.

The high-side line carrying hot high-pressure vapor from the compressor to the condenser; the smaller, hot line.

On some older systems, a window in the high side; bubbles/foam indicate an undercharge or non-condensables.

Liquid refrigerant returning to the compressor (which can't compress liquid), causing knock and damage; the accumulator helps prevent it.

Only Section 609-certified technicians may purchase refrigerant and service motor-vehicle A/C systems.

EPA-certified machines used to recover, recycle, evacuate, and recharge so refrigerant is captured and clean — required by Section 609.

Oil circulates with the refrigerant to lubricate the compressor; replace lost oil to the spec amount when replacing components.

Federal law (Section 609) prohibits it; R-134a/R-1234yf are greenhouse gases and R-12 depletes the ozone layer.

A high-voltage motor-driven compressor (no belt) that uses special non-conductive POE oil; never service with the wrong oil.

It is mildly flammable, so recovery/recharge machines and leak detection are designed and certified specifically for it.

Mixed or unknown refrigerant that can damage equipment; a refrigerant identifier checks purity before recovery.

The accurate way to charge: recover, evacuate, then add refrigerant by weight per the underhood label spec.

After evacuation, closing the valves and watching that the vacuum holds confirms the system is leak-free and dry.

Measures how deep a vacuum the system reaches during evacuation; a deep, stable vacuum confirms moisture removal.

Some vehicles use separate fans; a dead condenser fan hurts A/C high-side pressure even if the engine isn't overheating.

On some R-1234yf systems, transfers heat between liquid and suction lines to improve efficiency; account for it during service.

On a cold-soaked system, refrigerant moves to the coldest component; affects startup pressures and a few minutes' cooling.

The fitting that connects gauge/charging hoses to the service ports; depresses the Schrader valve to read or charge.

Match measured pressure to the refrigerant's chart to estimate temperature and judge subcooling/superheat.

Dried or wrong O-rings leak refrigerant; replace with the correct refrigerant-compatible O-ring and lubricate with A/C oil.

The fan that pulls outside or recirculated air into the HVAC case and pushes it through the system to the vents.

A resistor pack that sets the lower blower speeds by dropping voltage; when it fails, the fan often works only on high.

A failed blower motor resistor — high speed bypasses the resistor and still works.

A solid-state controller that gives variable, infinitely adjustable blower speed instead of fixed resistor steps.

Worn bearings or debris in the squirrel-cage fan; can overheat the resistor or module and burn out connectors.

A door, moved by an actuator, that mixes how much air passes through the heater core, setting the discharge air temperature.

Wrong-temperature air — air that won't get warm or cold, or hot on one side and cold on the other in dual-zone systems.

A door that directs blended air to the defrost, dash (panel), or floor outlets.

Air comes out of the wrong vents — for example, only defrost or only floor regardless of the setting.

A small electric motor that moves a blend or mode door to a commanded position based on the control head.

Often a stripped or obstructed blend/mode-door actuator hitting its stops.

A filter that cleans dust and pollen from air entering the passenger compartment; a clogged one weakens airflow at the vents.

Often a clogged cabin air filter or a restricted/blocked HVAC case inlet — with no electrical fault.

The dashboard control panel where the driver selects temperature, mode, fan speed, and A/C; it commands the actuators and blower.

Opens the compressor-clutch circuit when refrigerant pressure is too low (e.g., a leak), protecting the compressor from running dry.

Disengages the compressor clutch when high-side pressure is dangerously high, protecting the system.

Refrigerant charge: a low charge opens the low-pressure switch so the clutch can't engage — the most common cause.

Refrigerant charge (low-pressure switch), fuses/relays, the clutch coil, then the control input/request signal.

The electromagnet that, when energized, locks the pulley to the compressor shaft; an open coil leaves the clutch disengaged.

The small spec'd gap between the clutch plate and pulley; too wide and the clutch won't engage or slips.

An electromechanical switch that powers the compressor clutch (and often the cooling fan) on the PCM's command.

The control module enables the compressor only when conditions are safe (engine load, coolant temp, pressures) and may cut A/C at wide-open throttle.

A system that cycles the compressor clutch on a low-pressure switch to control evaporator temperature and prevent freeze-up.

Reports evaporator temperature so the control module cycles or modulates the compressor to prevent the coil from icing.

A climate system that uses sensors and a module to hold a set cabin temperature automatically by positioning doors and the blower.

Tells the ATC module the cabin temperature; often aspirated by a small fan for an accurate reading.

Reports outside temperature to the ATC module to help set blend, blower, and mode.

A photodiode on the dash that measures sunlight so ATC compensates for solar heat gain; a fault makes the system over- or under-cool.

Many climate-control heads enter a diagnostic mode that flashes fault codes and can sweep the actuators to test them.

Separate driver and passenger temperatures using two blend doors/actuators; a failed actuator gives one-side-hot complaints.

Moves the fresh/recirculate door at the blower inlet; a fault can cause weak cooling or fogging from the wrong air source.

Engine vacuum moves HVAC doors; a leaking vacuum hose or reservoir defaults airflow to defrost.

On vacuum-controlled systems, a leak sends all airflow to defrost — a built-in safe default.

Modern climate controls communicate over a data network; the control head sends requests and actuators report position digitally.

After replacing an actuator or battery, some systems must recalibrate door travel limits so positions read correctly.

Supplies battery power to the blower motor; a failed relay or burnt connector causes a no-blower condition.

Suspect the blower motor, its power/ground, the relay/fuse, or the control module — not just the resistor.

Some systems delay blower start until coolant warms (so cold air isn't blown in winter) using the ECT signal.

A sensor that reports system pressure to the PCM (replacing simple switches) for clutch, fan, and idle-up control.

The PCM raises idle speed when the compressor engages to offset its load and prevent stalling.

Older systems use vacuum motors to move mode doors; newer ones use electric stepper/actuator motors with position feedback.

Comfort circuits controlled alongside HVAC; rear-defogger grids clear glass via resistive heating on a timed relay.

A printed resistive grid on the rear glass that heats to clear fog/ice; a broken grid line leaves a stripe uncleared.

Selects recirculate, full cold, and high blower for fastest cooling by reusing already-cooled cabin air.

A potentiometer or hall sensor inside the actuator that tells the module the door's actual position for closed-loop control.

Points to a control-side fault — a stuck blend door or failed actuator — rather than the refrigeration components.

The PCM may disengage the compressor briefly at WOT to free up engine power — normal behavior.

On resistor systems, distinct speeds (low/medium/high); losing some-but-not-all speeds points to specific resistor coils.

The signature ASE format: judge each technician's statement separately as true or false, then choose A only, B only, both, or neither.

Good A7 diagnosis separates refrigeration faults (pressures/cooling) from control faults (doors/actuators/electrical) before replacing parts.

A single switch that opens the clutch circuit on both too-low and too-high pressure for protection.

Adds a third function to the binary switch — turning the cooling fan on at high pressure for extra condenser cooling.

Moves a door in precise increments without a separate position sensor; may need a relearn after power loss.

A blown fuse or bad ground can disable the blower, clutch, or controls; check power and ground before condemning parts.

After battery disconnect or actuator replacement, the module may need an initialization to recalibrate door positions.

Many systems mount the pressure switch on the receiver-drier/accumulator to sense system pressure for clutch control.

Low compressor speed and high-side pressure at idle, plus no ram air — the fan and idle-up keep performance up.

Heat issues point to coolant/heater core/blend door; cooling issues point to refrigerant/compressor/condenser — isolate which.

Selecting defrost usually overrides and runs the A/C to dehumidify; if it doesn't, check the compressor request circuit.

Cycle through panel, bi-level, floor, mix, and defrost and confirm air comes from the correct vents to test mode doors.

Reading desired vs. actual door positions, sensor temps, and pressures speeds diagnosis of control-side faults.

A small venturi or fan that draws cabin air past the in-car temperature sensor so it reads true cabin temperature.

The control module won't run the clutch with abnormal pressures, low voltage, or high engine load — preventing damage.

A blower or airflow fault (motor, resistor/module, clogged filter, case door) affects both — air isn't reaching the vents.

The PCM/relay turns the cooling fan on with A/C to keep condenser pressure down; a fault causes high-side overheating.

Verify the complaint, read pressures and sensor data, separate refrigeration from control faults, fix the cause, then retest.