The check engine light went on-took car to dealer, and they said fuel filler neck needed replacement. They would need to order part (about a week). Check engine light goes back off and stays off, yet above repair job not done yet. In your opinions, not surprising, or likely the dealer’s trying to scam me?
Dealers do not typically “scam you”. It all depends on what the check engine light code is, can be read at autozone or other auto parts store.
If you feel distrustful pay for a diagnosis at an independent shop that is recommended by others.
A frequent cause of the check engine light coming on is a loose gas cap. It will usually go off after the cap is put on correctly and that may be all that was wrong. If there were a leak in the fuel filler neck you would get the same code, but it would not go away.
The problem could be marginal or intermittent. It may come back without the replacement part. It is possible that was a loose gas cap, but, it may not have been that. It is not possible for us to know from here. You did not say what car, make or model or year you are talking about. Is it under warranty? If not why would you take it to a dealer?
Did the check engine go off, or did the dealer turn it off? If the dealer erased the code, it can take several days of driving for the engine computer to test everything and decide to turn the light on, again. Ask the dealer about your concern.