NVQ L4 Automobile AC Technician cheat sheet

R-1234yf is classified A2L (lower flammability); R-134a is A1 (non-flammable). A2L handling requires ignition-source control, ventilation, and leak-before-ignition awareness.

Oxygen-depletion asphyxiation risk exists with all halogenated refrigerants in confined spaces; exposure limits are set by national COSHH/OSHA-style regulations, not by refrigerant manufacturers.

Thermal decomposition of refrigerants on hot surfaces or flames produces hydrogen fluoride and carbonyl fluoride; never braze near an open system without nitrogen purge and PPE.

Frostbite and cold burns occur on skin contact with liquid refrigerant at approximately -26 to -50 °C; PPE includes cryogenic gloves, face shield, and eye protection.

Recovered refrigerant is hazardous waste until reclaimed to AHRI 700 purity; venting to atmosphere is prohibited in most jurisdictions and carries penalties.

Refrigerant cylinders must be hydrostatically tested (typically every 5 years) and never filled above the marked tare weight plus refrigerant capacity.

A2L refrigerants require a refrigerant detection sensor and, in some vehicles, an automatic shut-off valve; technicians must never bypass these safety systems.

First aid for liquid refrigerant skin contact: flush with lukewarm water, do not rub, and seek medical attention for blistering or eye exposure.

Recovery removes refrigerant; recycling cleans it on site (oil/particulate removal); reclamation returns it to AHRI 700 new-product specification and requires laboratory analysis.

Recovery machine hoses and cylinders must match refrigerant type; cross-contamination of R-134a and R-1234yf renders both unreclaimable and is a common workshop failure.

Recovery cylinder fill is limited to 80% of water capacity by weight; use the tare weight plus refrigerant capacity marked on the cylinder to calculate maximum fill.

Recovery must continue until the system reaches approximately 0 psig (or the machine's vacuum threshold); residual pressure indicates incomplete recovery.

Non-condensable gases (air) in a recovered cylinder raise head pressure; purge via the machine's air-purge function or by venting a small amount of vapor from the top of the cylinder.

Cylinder valves must be closed and caps fitted during transport; cylinders are stored upright, below 50 °C, away from oxidizers, and secured against falling.

Refrigerant identifier testing is required before recovery into a shared cylinder; contaminated refrigerant must be sent for reclamation, not blended or topped up.

Recovery machine filters and dryers require scheduled replacement; saturated desiccant allows moisture and acid to pass into the recovered refrigerant.

Static pressure approximates saturation pressure at ambient temperature: R-134a at 20 °C is about 5.7 bar (83 psi) absolute; use PT charts, not guesses.

Low-side pressure too low with high-side too high and a hot compressor indicates a restriction (expansion valve, orifice tube, or receiver-drier).

Both sides low with a cycling compressor clutch indicates low charge; both sides high with poor cooling indicates condenser airflow or overcharge.

Electronic leak detection (heated diode or infrared) is more sensitive than UV dye; UV dye is used for slow leaks and requires a blacklight inspection after operation.

Nitrogen pressure testing at 10-15 bar with a pressure-decay check locates leaks without introducing moisture; never pressure-test with oxygen or compressed air.

A vacuum hold test (vacuum decay) verifies system integrity; a rise in pressure during isolation indicates a leak or residual moisture boiling off.

Compressor clutch not engaging: check refrigerant pressure switch, clutch coil resistance (typically 3-5 ohms), relay, fuse, and PCM command before condemning the compressor.

Temperature stratification across the evaporator (measured with thermocouples) indicates a blocked expansion valve, distributor, or low charge.

Always recover refrigerant, cap open lines immediately, and replace the receiver-drier or accumulator whenever the system is opened to atmosphere.

Use O-ring seals compatible with the refrigerant and oil: HNBR or neoprene for R-134a, and specified materials for R-1234yf; mineral oil seals fail with HFCs.

Torque values for flare and O-ring fittings are specified by the manufacturer; over-torque deforms the O-ring and causes leaks, under-torque allows vibration loosening.

Add the specified oil quantity to a replaced component (compressor, evaporator, condenser) and subtract the amount drained from the removed part.

PAG oil is hygroscopic; keep containers sealed, and never mix PAG with mineral oil or PAG of a different viscosity without manufacturer approval.

Flush the system only with approved flushing agents and nitrogen; residual flush solvent contaminates the new oil and damages the compressor.

After component replacement, evacuate to below 500 microns and hold for at least 10 minutes; a rising vacuum indicates a leak or moisture.

Compressor replacement requires verifying oil balance, clutch gap, belt alignment, and that the new compressor is pre-charged with the correct oil amount.

Evacuation removes moisture and non-condensables; a target of 500 microns (0.67 mbar) or lower is typical, with a hold test of at least 10 minutes.

Moisture in a system forms acids with PAG oil and degrades the compressor; a vacuum decay that plateaus then rises suggests moisture, not a leak.

Charge by weight using a refrigerant scale; charge by pressure is unreliable because it varies with ambient temperature and airflow.

Subcooling (condenser outlet temperature minus saturation temperature at high-side pressure) targets 5-10 K; low subcooling indicates undercharge.

Superheat (suction temperature minus saturation temperature at low-side pressure) targets 5-10 K at the evaporator outlet; high superheat indicates undercharge or a restriction.

Performance test conditions: cabin temperature set to max cold, blower on high, windows open, engine at 1500-2000 rpm, ambient above 20 °C; record vent temperature (target typically 4-10 °C).

A2L systems may require a leak-detection sensor calibration and system self-test after charging; verify no fault codes remain before handover.

Commissioning records include refrigerant type, charge weight, oil type and quantity, vacuum level, vent temperature, and ambient conditions.

The HVAC control module reads inputs (ambient temperature, evaporator temperature, pressure sensor, solar load, cabin temperature) and commands the compressor clutch, blower, and blend doors.

A 3-wire pressure sensor outputs a voltage proportional to pressure; a 2-wire switch is a simple open/closed device. Diagnose by measuring voltage against the manufacturer's chart.

Evaporator temperature sensor prevents evaporator icing; a failed sensor typically causes the compressor to cycle rapidly or not engage.

CAN bus faults can disable the AC compressor; scan all modules for communication codes before replacing components.

Blend door actuators are stepper or DC motors with position feedback; a stripped gear or lost calibration causes temperature stratification between vents.

PWM-controlled variable-displacement compressors use a control valve driven by the module; measure duty cycle and current, not just clutch voltage.

A2L vehicles use a refrigerant sensor that communicates over LIN or CAN; a triggered sensor may command the HVAC to close the fresh-air door and disable the compressor.

Use a wiring diagram and measure voltage drop under load; a 0.5 V drop across a ground or connector indicates excessive resistance.

Air distribution modes (face, floor, defrost, blend) are controlled by mode door actuators; incorrect mode is usually an actuator, linkage, or calibration fault.

Cabin air filters (particulate or activated carbon) restrict airflow when clogged; a pressure drop across the filter above specification indicates replacement.

Recirculation mode reduces cooling load but increases humidity and fogging risk; fresh-air mode is required for defogging.

Defrost performance depends on blend door position, heater core flow, and AC operation (AC dehumidifies in defrost mode).

Cabin air quality sensors (AQS) automatically switch to recirculation when high pollutant levels are detected; a failed sensor may leave the system stuck in recirculation.

Refrigerant odor or musty smell indicates evaporator microbial growth or a leaking heater core; clean and disinfect the evaporator case and verify drains.

Airflow measurement (anemometer at vents) verifies blower performance; low flow with a good blower indicates a blocked evaporator, filter, or duct.

Cabin comfort depends on radiant load, humidity, and air velocity; vent temperature alone does not confirm comfort performance.

R-1234yf has a GWP of about 4 versus R-134a at about 1430; it is classified A2L and requires ignition-source control and leak detection.

R-744 (CO2) operates at much higher pressures (up to 130 bar) and requires dedicated equipment, high-pressure-rated hoses, and different oil (PAG for CO2).

R-1234yf is mildly flammable; service equipment must be rated for A2L and include ventilation and leak detection.

R-152a has been used in some markets as a low-GWP alternative; it is also A2L and requires similar safety measures.

Electric vehicle heat pump systems use the AC circuit for both cooling and heating; service procedures differ and high-voltage safety training is required.

Refrigerant blends (e.g., R-404A, R-410A) are not drop-in replacements for R-134a; they have different glide and pressure characteristics.

Retrofit from R-134a to R-1234yf requires flushing, new oil, new seals, and updated labels; mixing refrigerants is prohibited.

Regulatory phase-downs (EU F-gas, US AIM Act) drive adoption of low-GWP refrigerants; technicians must verify refrigerant identity before service.

Record refrigerant type, charge weight, oil type and quantity, vacuum level, leak test result, and vent temperature on every job card.

Explain the difference between a leak repair and a recharge to the customer; a recharge without leak repair is a temporary fix and should be documented.

Warranty and regulatory records must be retained per local requirements (often 3-5 years) and made available for audit.

Supervisors must verify technician competence for A2L handling and ensure equipment calibration records are current.

Customer complaints about odor or insufficient cooling should be documented with measured vent temperature and ambient conditions, not subjective descriptions.

Refrigerant purchases and disposals must be logged to reconcile inventory and demonstrate compliance with environmental regulations.

Handover should include a written explanation of work performed, parts replaced, and any remaining faults or recommendations.

Workshop supervision includes toolbox talks on A2L safety, emergency procedures, and correct PPE use, with attendance recorded.