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ASE P2 TEST PRIMER: Heating, Ventilation And Air Conditioning (HVAC)


Loss of refrigerant is the most common cause of A/C cooling problems.


The major A/C system components include the compressor (compresses and pumps refrigerant through the system), compressor clutch (engages/disengages the compressor), orifice tube or expansion valve (controls refrigerant flow into the evaporator), condenser (mounted in front of the radiator to cool the refrigerant), evaporator (mounted in the HVAC unit to cool air entering the passenger compartment), accumulator or receiver/drier (serves as a refrigerant reservoir and filter to remove moisture), suction hose (located between evaporator and condenser), high-pressure hose (located between compressor and condenser), and a second high-pressure hose or metal line between the condenser and evaporator.

Related parts found in most A/C systems include a low-pressure safety switch (prevents compressor from engaging if refrigerant is low), HVAC fan, fan resistor block or blower control module, automatic climate control module, interior air temperature sensor, sunload sensor (allows A/C system to increase cooling when dash is in direct sunlight), refrigerant (R-134a for 1995 and newer vehicles), refrigerant hoses, fittings and o-ring seals, and heater core.


Loss of refrigerant is the most common cause of A/C cooling problems. Leaks can occur in hoses, hose or line fittings, the condenser or evaporator. Leaks can be found by adding ultraviolet leak detection dye to the refrigerant, or by using an electronic leak detector tool. Compressor failures can occur as a result of low refrigerant, the wrong refrigerant, loss of compressor oil or the wrong compressor oil or refrigerant contamination.

If a compressor has failed, the condenser should be flushed with an “approved” flushing chemical to remove contaminants, or replaced if it is a parallel flow condenser or one with very small passages that cannot be flushed. The orifice tube and receiver/drier or accumulator should also be replaced.


Different types of compressors require specific types of oil. Older R-12 A/C systems require mineral oil, while newer R-134a systems require a specific type of PAG oil. Older R-12 systems that have been retrofitted to R-12 can use POE oil or PAG oil as specified by the compressor manufacturer.

When major A/C system components are replaced, the A/C system must be vacuum-purged to remove all traces of air and moisture before it is recharged with refrigerant. The correct amount of refrigerant and compressor oil must be added to the system for proper operation.

A related HVAC item that may need to be replaced on newer vehicles is the cabin air filter (located behind the glove box or at the base of the windshield).


Required to pass this section of the P2 test:
1. Identify major A/C system components.
2. Identify component function and common reasons for replacement.
3. Identify related items, including refrigerants, service chemicals and tools.
4. Provide basic use, maintenance, installation and warranty information.

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