Ours are 7 years old. Have run flawless, never a hiccup. One of them quit making hot water. On inspection, neither LED is lit (no flashing trouble code). The unit has power, the master trip is working (as in I can trip it out with the button and the red LED comes on, then reset and the light goes out) but the LEDs on the main board are not on. I've checked the two fuses where the power comes in and the fuse on the main curcuit board. They are good. Tankless is very rare in my area, finding a reputable tech is going to be difficult.
Call the Takagi help line- let them talk you through it. It may be a whole control-board failure or something a simple as a loose or corroded connector. It's likely that they still stock the board-level assemblies as a spare part if it comes to that.