The sticker is required by Federal law, but very few converted vehicles ever get it. Even when the sticker is there, it really doesn't tell you much. There are WAY too many people that will put whatever refrigerant for whatever reason in whatever car violating Federal law and don't think twice about it. For this reason, you best use a Refrigerant Identifier if you're not 100% sure of the vehicles refrigerant history.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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