Refrigerant Temperature-Pressure Chart (R-12 / R-134a / R-1234yf)

Combined P-T chart (saturation pressures)

How to use a P-T chart

• Static (system off): low-side and high-side pressures should both equal the saturation pressure of the refrigerant at ambient temperature. Example at 75°F with R-134a: both gauges should read ~71 psig. If they're different, system has air, oil, or moisture mixed in.
• Running (system on): high-side pressure should match saturation at the condenser-outlet temperature (typically 30-50°F above ambient). Low-side should match saturation at the evaporator temperature (typically 30-45°F).
• Diagnose by working backward: measure low-side gauge → look up saturation temp → that's the evaporator temp. If gauge says 30 psig with R-134a, evaporator is at 32°F (icing point — investigate).
• Subcooling and superheat: difference between actual temperature and saturation temperature at that pressure tells you whether the refrigerant is fully liquid (subcool) at the condenser outlet or fully vapor (superheat) at the evaporator outlet. Healthy systems: 10-20°F subcool, 8-15°F superheat.
• Pressures are gauge (psig / kPa above atmospheric). Add 14.7 psi (101 kPa) to get absolute. A reading of "0 psig" = atmospheric pressure.

Refrigerant identification at a glance

Higher operating pressure

R-12 ≈ R-1234yf > R-134a. At 100°F: R-12 = 174 psig · R-1234yf = 162 psig · R-134a = 116 psig. R-134a runs 25-30% lower than R-12 — you cannot simply substitute it without modifying components.

Service port match

R-12 = brass flare. R-134a = 13/16 mm Schrader quick-connect. R-1234yf = 14/17 mm quick-connect (different from R-134a). Cap colors: R-134a blue/red, R-1234yf gray.

Underhood label

Federal law requires every car to have a refrigerant ID + charge weight label under the hood. ALWAYS check before service.

Cylinder colors

R-12 = white. R-134a = light blue. R-1234yf = white with red shoulder. Don't rely on color alone — counterfeit cylinders exist.

Why R-12 is banned

• R-12 (CFC-12, dichlorodifluoromethane) has an Ozone Depletion Potential of 1.0 — the worst grade. It was identified in 1974 as a key driver of stratospheric ozone destruction over Antarctica.
• Production was banned in developed countries on January 1, 1996 under the Montreal Protocol (1987).
• You may still recharge an existing R-12 system with recycled R-12 from reclaimed stock if you're EPA-certified — but it's expensive (often $50-100/lb retail) and shrinking.
• Retrofit kits convert R-12 systems to R-134a — typically requires a new receiver-drier, hose / O-ring replacement (R-134a leaks through R-12 hose material), oil flush (mineral → PAG), and installation of new service ports.
• R-1234yf retrofitting is generally NOT viable for older R-12 cars due to dramatically different system pressures and lubricant requirements.

GWP / ODP comparison

Boiling points (1 atm)

R-12

−21.6°F (−29.8°C)

R-134a

−15.0°F (−26.1°C)

R-1234yf

−20.4°F (−29.1°C) — slightly mildly flammable (A2L safety class)

R-744 (CO₂)

−109.3°F (−78.5°C) sublimation; behaves as transcritical fluid in A/C use

Critical points (above this = supercritical, no liquid)

R-12

T_c = 233.6°F (112°C), P_c = 597 psia

R-134a

T_c = 213.9°F (101°C), P_c = 588 psia

R-1234yf

T_c = 201.9°F (94.7°C), P_c = 488 psia

R-744 (CO₂)

T_c = 87.8°F (31.0°C), P_c = 1071 psia — runs transcritical in A/C operation