use some WD40 in the purge valve, it may free it up, I had to do that on my old W210 too, Benz have been using the same kind of valve forever and a squirt of WD40 helps it when it becomes lazy.

a sticky/lazy purge valve sets that code too.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)