We bought a house a few years ago that had a tankless
water heater. A couple of months ago, it started cycling on and off all the time. I tried calling a few local repairmen and every single one said “no thanks, we don’t service tankless”. One even told me that I should go on the manufacturers website and see who the authorized repair companies were. Which was funny, since that’s how I got most of those phone numbers.
The good news is that the water heater could still work in limp mode. It couldn’t even regulate the temperature but it would try its best to heat it up to 120 (and no greater). So it basically didn’t impact us too much. It wouldn’t last forever in limp mode but it could possibly last months in this mode.
Anyway I ended up on tech support with the manufacturer. They said I needed a new logic board. I popped open the cover and saw that some capacitors were definitely bulging. A common problem with this model apparently. So I agreed and they sent new one out. Unfortunately we were out of warranty, so I had to pay full price for it. And then they shipped the wrong one, which they corrected but also forgot to tell me I needed a program chip, which got shipped out after that. I finally had everything I needed after a few weeks of waiting.
Swapping the board was no worse than swapping a motherboard. Even easier, actually, since every connector was unique and there was no way to plug it in wrong. I went through the process of maybe 20 steps to get everything flashed and configured correctly (thankfully well detailed in the documentation) and was up and running in less than 30 minutes.
But I ended up spending almost as much as a brand new tanked water heater would have cost. The house wasn’t set up for a tank, so the installation would have been more than I felt ready to do (or pay for), but it still kinda irked me. The tankless water heaters are nice but everything is way more expensive.