First, I'm not a Mercedes mechanic or a mechanic at all, really.

In my experience, when the can bus goes down, it's almost always one of a few things.

1) Broke/grounded wire. A broke or bad connection can be hard to diagnosis with a multimeter, you need to load the circuit. But if you're seen multiple faults, this is less likely.

2) A module on the can bus is bringing down the entire network. On a crank/no start where there's no CAN activity or faulty activity (i.e. low amplitude or the wave forms don't match) I've had a lot of success unplugging modules and seeing if the network comes back. I've seen many an ABS module take down a CAN network causing a no start. You pull the connector and brake fluid starts pouring out.

3) ECU. It happens.

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1997 E300 Diesel - 222K miles

Good to replace K40 "just because." Cheap and can cause all manner of electrical weirdness.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife