ehm, you can have the Febi tensioner replaced with a genuine MB tensioner by someone who knows what they are doing, either you or a tech familiar with the procedure.

Hopefully, you are confronting the shop that performed the work using aftermarket parts while claiming the parts were genuine MB parts. Always best to have a written contract/proposal that both parties sign so there is no misinterpretation about specific details.

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Fred Hoelzle