Thanks for the follow-up answer.
Damn, I have replaced quite a few oxygen sensors on mid '80s Mercedes 4's and 6's engine cars and never had to use such a tool. I have to admit I hadn't even heard of it.
Ha,ha,ha! Ah well, you never stop learning. Such a tool makes a lot of sense in case of the 400E. No space in there.
Thank you very much again, your advice has been very educational and helpful. I'll let you know how things went.