I have an error code P0340, camshaft position sensor A Circuit bank 1 or single sensor. Where is the camshaft position sensor located on a GL350 Bluetec engine?

Assuming you have the OM642 V6 in that model (99% sure you do)...

The cam sensor is on the passenger side, on top of the cam cover, between cylinders 1 and 2. It's the only electrical thing near there besides the injectors and glow plugs.

(1 is the front right, 2 is the center right, 3 is the rear right, 4 is left front, 5 is center left, 6 is rear left).

You will have to undo the air pipe on the right side.
Once you have undone the air pipe then loosen the bolts for the air box and just pivot it out to the side and pull off the foam padding.

One bolt one plug pull it out. Get a factory one from the dealer don't mess with aftermarket, the engine cannot run without this. The factory one comes with a new o-ring.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks!
I ended up taking it to Mercedes after a incident where it would not start. Mercedes found that the timing chain was causing the problem after they performed the emissions programing upgrade. Luckily Mercedes is covering the repair under warranty. This totally surprised me since it is a 2014, 7 year old.