One of the biggest mistakes I ever made was retroffiting my 300SD to use R134a. However, I did that back when I didn't know anything about these A/C systems and how easy it is to get the 609 Certification. I have since received that certification and purchased several pounds of R12, some of which I used to recharge the car that now belongs to Oilbrnr.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver