Chris Back

Hi all. The Check Engine Light recently came on on my 2012 C250 (80k miles). I was also experiencing the long crank on start, just an extra second or two.
I pulled the code off the generic OBD II reader and found P0340. Given the simplicity of replacing the sensor, I replaced it, first on the intake side, then when that didn't resolve the issue, replacing the exhaust side. Ultimately neither fixed the issue.
Numerous Google search led me here, where it seems common for this to be due to a number of things:
a) timing chain adjuster
b) timing chain stretch
c) camshaft adjuster failed
I pulled the valve cover off, which was much more difficult than I would have expected. I manually rotated the engine to get the crankshaft pulley at 0 degrees, and it appears to be that the camshaft adjusters are lined up correctly as well, at least the marks on the larger part in front of the timing chain. The marks on closer to the camshaft were not aligned, which seems ok to me since that is what the adjuster would be adjusting. Regardless, a couple of pics

exhaust cam adjuster

intake cam adjuster

The timing chain doesn't appear loose at all, i.e. no play in it. So that leaves me with trying to tell if the camshaft adjusters or timing chain adjuster is bad. How can I test those?

Thanks for any help,
Chris