A fidicuary duty arises where a person agrees or is statutorily bound to act in the financial best interests of another. Typically, the relationship is considered an agency, whereby the principal authorizes the agent either by express or implied contract (actual authority), by manifesting the existence of the agency to third parties with whom the agent may deal (apparent authority), or because the agent's acts are so substantially done for the benefit of the principal, and with the principal's knowledge, that it is fair that the principal be bound by them (inherent/ostensible authority).
A car dealership service department has no fiduciary duty to a customer, mainly, because neither the customer, as principal, nor the service department ever manifests the existence of any agency authority. Moreover, and perhaps more importantly, the service department is already the agent of the manufacturer with regard to warranty repair activities, and so it cannot act as agent for the customer without creating a conflict of interest.
Thus, there is no fiduciary duty of a car dealershp service department to a customer.
However, a dealership's failure to inform a customer about the existence of known warranty defects could certainly be negligence, if the failure to warn were to damage the customer's vehicle. It is also a breach of contract and/or of the implied covenant of good faith, because the customer contracts with the dealer to repair the vehicle and failing to do so is a breach.
Why a service department would deny the existence of a recall, while knowing that the manufacturer will pay for the repair is beyond me. If it did, that would violate its duty to the manufacturer and make the dealership more likely to be found directly liable for damages consequential to any product liability action.