Turn on hot water - occasionally comes out cold

[Wires001]

I have a Stiebel-Eltron tankless water heater installed around 2019. Occasionally (like once every 2-3 weeks) I can turn on one of the two bathroom sink faucets or kitchen sink faucet in the house and it stays cold - it doesn't matter how long I run it. But if I go to a shower, it comes out hot immediately. Then if I run back to the sink, it's hot now too. One time last week I was taking a shower and it turned cold after a very short time. I turned the water off then back on and it came out hot.

Any idea what could be causing this?

Maybe a flaky pressure sensor?

 

[Fitter30]

Has the heater been service every year? With vinegar solution run through it? Next time turn on a sink or tub for more flow see if that helps or before doing doing anything look at flow rate at heater display.