when exactly did (R134a) air con gas become compulsory in cars?

It was not that r134a be came compulsory. It was that 12R was made illegal and baned. Of the replacement refrigerate 134R was the safety to use in the vehicle. Some were in the late 90. I have a 93 with 12R and a 94 with 134R. Of course I have not keeping up with the laws on repairing vehicle in Florida there may now be a law as to which refrigerate you have to use. I just found out that it is against the law to have a secrete compartment in you car or boat in Florida. Now were would you keep you legal gun to keep it from being stolen.

1994

R-134a first appeared in the early 1990s as a replacement for Dichlorodifluoromethane (R-12), which has significant ozone depleting properties.[3] R-134a has been atmospherically modeled for its impact on depleting ozone and as a contributor to global warming. Research suggests that over the past 10 years the concentration of 1,1,1,2-tetrafluoroethane has increased significantly in the Earth atmosphere, with a recent study revealing a doubling in atmospheric concentration between 2001-2004.[4] It has insignificant ozone depletion potential (ozone layer), significant global warming potential (GWP100 = 1300) and negligible acidification potential (acid rain). 1,1,1,2-Tetrafluoroethane is slowly converted to trifluoroacetic acid through a radical reaction in the upper atmosphere and leads to a detectable amount of several ng/L in acid rain.[5]

Yep Everything he said It was about 93-94 when they changed it over.

Short answer: It was 94 or 95.