This paper summarizes hot surface ignition characteristics of R134a and R1234yf automotive air conditioning fluids on typical under-hood automotive surfaces that possess sufficient heat to ignite flammable or combustible fluids. It further investigates the effect, if any, that these two different fluids may have on the propagation of a fire in two identically equipped vehicles under similar test conditions. This testing, in part, is in response to the United States Environmental Protection Agency's proposal which seeks comments concerning the proposed replacement of the current R134a air conditioning refrigerant with R1234yf.R134a is currently regarded as the global choice for automotive air conditioning systems however the EPA classifies it as an ozone-depleting substance (ODS) and is tasked with proposing and reviewing alternatives that do not contribute to stratospheric ozone depletion. R1234yf refrigerant is classified as a non-ozone-depleting gas by the EPA and has been proposed as an acceptable alternative to R134a through the authority of the Clean Air Act and the Significant New Alternatives Policy (SNAP) program. The SNAP program sets forth criteria for the implementation of R1234yf refrigerant and has indicated that the proposed alternative refrigerant does not have to be risk free to be found acceptable for use. European Union (EU) countries have developed Directive 2006/40/EC that mandates a 100-year CO₂ Global Warming Potential (GWP) of less than 150. R134a is nearly 10 times this level with a rating of 1430 GWP. R1234yf has a GWP rating of 4.0 or approximately 0.3% of the current R134a refrigerant.Hot surface ignition testing of R134a and R1234yf mixtures with PAG 46 oil were evaluated in the first part of this paper. Typical under-hood automotive surfaces at maximum operating temperatures were established, along with a functional AC system to evaluate the ignition characteristics of the refrigerant and lubricant mixtures. Tightly controlled test parameters and conditions were utilized to minimize test-to-test variation. The results obtained in this testing were compared to the published autoignition properties of the refrigerants and other automotive fluids. The second part of this paper assesses the potential effects that the changing from R134a to R1234yf may have on fire propagation in a motor vehicle.