In a beach town, salt air slowly corrodes anything made of metal. A tankless is no different. The venting, the gas connections and the fittings take the salt year after year, while hard water works on the unit from the inside.
When a code shows up, it can come from either side. It might be scale in the heat exchanger making the unit run hot, or it might be corrosion on the venting or connections. The first visit is about finding out which.
From there you get a straight answer. If it's a descaling or a part, that's what you'll hear. If the unit is worn out, you'll get your options for a replacement chosen with the salt air in mind. You make the call.