Buy a cheap gas cap. Take the o ring off and put it on. Or flip your gas cap ring over. I've done this to several cars for EVAP codes. If the gas cap o ring is good enough, it will work to tighten up the seal. Flipping the o ring on mine worked for a while. Eventually, I took a seal off a new gas cap. No EVAP codes have appeared for two years.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)