So I let my tank level drop to below 1/4 and my SES light came back. I haven't read the codes yet, but assuming they are the same, do you think lower fuel levels can cause the engine to run lean? To me, that sounds like a weak fuel pump.
I'll get a fuel pressure gauge and test. Is there a specific type that is compatible with the m54?
I also like the idea of changing the fuel filter, inexpensive and a good starting point.