I had a replacement central locking pump fitted to my Mercedes-Benz SL55 AMG by a local authorised dealer. However, after just eight months, the part failed again. The dealer is now refusing to replace it under their genuine parts warranty as they claim that it has failed due to water ingress.

Water was found in the boot but they are only surmising that this water caused the failure. They told me they do not have authority to open the pump and refuse to send it back to Mercedes-Benz for them to make an accurate assessment. Do they not have an obligation to prove that water ingress caused the pump failure and not just surmise that it did?