Hello & Welcome to ja.com..
Your words under 'already tried'...... "Vehicle will try to start ,but won't even turn over" are somewhat contradictory -- but I think I know what you mean ;))
The problem you describe though is a fault with the immobilser system - indicated by the message "start error" - the aam unit is not recognzing the signal from your key.
Try another key - as it could be a fault with the one key - but to be honest with you , its usually vehicle end - and the bad news is, there is no way at all round this, other than contacting your Benz dealer and having them connect up the diagnostic machine / computer to see whats gone wrong.
I feel at this point it will be the receiver unit on the van - but there is nothing at all you can do on a diy basis Im afraid.... you are going to have to call it in.
I Sincerely XXXXX XXXXX have helped you -- Please ask if you need further help -
Kind Rgds, MIKE
PS~Please hit ACCEPT if you are happy with my answer/advice given~ Thank You ;)
Any & All Bonuses Are Gratefully Received ~ Thank you again.
Please leave some FEEDBACK if you have a mo.