In the heat of an El Cajon summer, the water coming into the house is already warm and a tankless barely has to try. Then the first cold mornings show up and everyone hits the shower at once. A unit that's been coasting all summer suddenly has real work to do, and that's often when a code starts flashing.
Hard water makes it worse. Scale builds inside the heat exchanger over the years and restricts flow. The unit runs hotter than it should, and eventually it shuts down to protect itself.
We'll get someone out to find out which it is, a descaling, a part or a unit that's done, and you'll hear it in plain English. If it's time to replace, the new one gets sized to the people who actually live in the house now.