MercedesMercedes-BenzGermany, 1924 > present197 models
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says that it will continue to use R134a refridgerant in its vehicles after it found that the new R1234yf refrigerant could be flammable in rear world conditions. 

R1234yf has been tested in the lab and been found safe, but Mercedes conducted its own independent tests and found that in a severe collision where the refridgerant line is severed, the hot engine compartment could cause the material to combust. Mercedes admits that the tests that it performed go beyond the legally mandated tests for R1234yf. However, it says that the restults of its real world tests are reproduciable in the real world. 

R1234yf is meant to be the new standard for refridgerants in cars. It has a Global Warming Potential rating of 4, compared to 1430 from R134a. On the scale, lower scores are better, which makes R1234yf 357.5 times cleaner than R134a. R1234yf has an atmospheric lifetime rating of 11 days compared to 13.5 years for R134a.

While different equipment is needed to fill vehicles with R1234yf than R134a, they are replacable within an actual vehicle. That means that cars that currently use R134a can switch to the new material and vice versa with no problem. However, cost is a major factor. Right now, R1234yf costs about ten times more than the same  volume of R134a. This is because there are still few factories producing R1234yf. R134a has been used in vehicles since the 1990s. 

Daimler has reported its results to the relevant environmental authorities about its findings.