Never heard of battery disconnection doing anything for hard fault codes.
I have a connector and an ignition coil coming in the mail today, then I'm going to go to where the car is, try to get cylinder #8 to start firing, unplug #8 injector if I can't, and drive it 40 miles home. Would actually be a couple of miles shorter and 20 minutes quicker if I can get it up to freeway speed, but I have no desire to scream down the freeway for 30 miles at 55mph in 2nd gear. 3rd gear maybe, but not 2nd.
I'm not really able to do much in the seller's driveway, but I did get permission to yank the #8 spark plug. The electrical connector at the right front of the tranny, and any oil wicking to the TCU, will have to wait for tomorrow.

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'79 300SD W116
'86 190E 2.3-16V W201
'92 300D 2.5 W124
'99 E320 4MATIC S210