There's two. They are in the block inside the chain guide covers. I don't remember what parts have to come out to get to them but I think it's between the oil distribution unit that brings the oil to the cam and the block. So: it's definitely an investment to get to the valve. But possibly you can test with just the valve cover removed. If the problem on the bank one you might be able to determine something through the oil fill port.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)