In a beach town, anything metal is slowly losing a fight with the air. 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. Scale in the heat exchanger making the unit run hot, or 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's had its run, you'll get your options for a replacement that can stand up to the salt air. You make the call.
