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

Saturation pressure-temperature data for the three automotive refrigerants. Use this chart to verify a system is correctly charged — the high-side pressure should match the saturation pressure at the condenser-outlet temperature, and the low-side pressure should match saturation at the evaporator temperature. A negative number means the saturation pressure is below atmospheric (vacuum) — only relevant in winter / cold-soak diagnostics.

Combined P-T chart (saturation pressures)

Temp °F	Temp °C	R-12 (CFC, legacy)		R-134a (1995-2016)		R-1234yf (2013+)
Temp °F	Temp °C	psig	kPa	psig	kPa	psig	kPa
-40	-40	-11.0	-76	-14.7	—	-12.6	-87
-30	-34	-7.7	-53	-12.4	-86	-9.2	-63
-20	-29	-3.2	-22	-9.5	-66	-4.3	-30
-10	-23	2.4	17	-5.9	-41	1.4	10
0	-18	9.2	63	-1.4	-10	7.6	52
10	-12	17.1	118	4.1	28	15.1	104
20	-7	26.5	183	10.7	74	24.0	165
30	-1	37.7	260	18.4	127	34.4	237
40	4	50.7	350	27.5	190	46.7	322
50	10	65.5	452	38.0	262	60.7	419
60	16	82.4	568	50.0	345	76.7	529
65	18	91.6	631	56.7	391	85.4	589
70	21	101.5	700	63.7	439	94.6	652
75	24	111.9	772	71.2	491	104.3	719
80	27	122.9	847	79.0	545	114.5	790
85	29	134.5	928	87.4	603	125.4	865
90	32	146.8	1012	96.4	665	136.9	944
95	35	159.8	1102	105.9	730	149.0	1027
100	38	173.5	1196	116.0	800	161.7	1115
105	41	187.9	1296	126.6	873	175.1	1207
110	43	203.0	1400	137.7	949	189.2	1305
115	46	218.9	1509	149.5	1031	204.0	1407
120	49	235.6	1624	161.9	1116	219.5	1514
125	52	253.1	1745	174.9	1206	235.9	1627
130	54	271.3	1871	188.6	1300	253.0	1745
135	57	290.5	2003	203.0	1400	271.0	1869
140	60	310.5	2141	218.0	1503	289.9	2000
145	63	331.3	2284	233.8	1612	309.6	2135
150	66	353.1	2435	250.4	1726	330.3	2278
155	68	375.7	2591	267.8	1846	351.9	2427
160	71	399.4	2754	286.0	1972	374.5	2583

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

Refrigerant	ODP (vs CFC-11)	GWP-100	Atmospheric lifetime	Status
R-12 (CFC-12)	1.0	10 900	100 years	Banned — Montreal Protocol 1996
R-134a (HFC)	0	1 430	13 years	Phased out for new vehicles in EU 2017+, US 2021+
R-1234yf (HFO)	0	4	11 days	Current OEM-required for new vehicles
R-744 (CO₂)	0	1	N/A (natural)	Mercedes EQ-class, some VW EVs (very high pressure)
R-152a	0	124	1.4 years	Niche, mildly flammable; some heat-pump applications

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

Notes

Data from NIST REFPROP (R-134a, R-1234yf) and ASHRAE 1991 (R-12). Pressures rounded to 0.1 psig / 1 kPa.
Negative values for R-134a / R-1234yf at very low temperatures indicate the saturation pressure is below atmospheric — system is in vacuum.
For the broader service procedure, normal operating pressures, oil compatibility, and diagnostic patterns, see the companion Automotive A/C Reference Guide.

Was this article helpful?