I believe your 400E has the same fan control strategy as my 350SDL... sometimes pulling the plug off starts the fans, sometimes it does not. Sometimes the engine has to be somewhat warm first. If you short the 2 pins together the fans should always come on.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.