If you aren't getting any error codes, then the heater is probably doing its job. Your problem is probably somewhere in your plumbing system. The most common reason you get this sort of problem is due to cold water mixing in the hot water system. There are a number of causes for a "cross connection" :

-Faulty check valve
-Missing check valve on reciculation return line
-Faulty shower cartridge
-Thermostatic mixing valve set too low
-Faulty thermostatic mixing valve
-Faulty anti-scald valve

The easiest way to confirm a cross connection is to: 1) shutoff the cold water entering the tankless heater 2) open hot water valves around the house. If you still have water flowing at your hot water fixtures, then you have a cross connection.