Bosch tankless GWH 2400 NG temp goes high and low

[JIM77]

Bosch gwh 2400 es ng temp will not hold setting. example 108 deg set will then blink and water goes to very high then must hit a limit and goes to less than 108 . This continues until unit is shut off and reset. It may work a week then returns to this flucuation. The unit was descaled and the hot water outlet temp sensor replaced. Bosch support has been no help what ever. They want a licensed plumber here to even suggest a fix. I have been a contractor to install electrical and plumbing for many years and i followed Bosch specs to the tee and did all maintenance as required. If i need to pay to have a no-knowledge guy here to take a call i will scrap the Bosch and go else where. Can anyone suggest a fix? The control unit is about $300 and not a sure thing. Is there a sofware fix?

 

[Jadnashua]

Check all connections and wires for possible corrosion or looseness. The temperature sensor will often have a chart indicating the resistance for a certain temperature...you want a quality ohmmeter to verify that, but a cheap one will still vary as the temperature changes. The manual often will give the testing procedures. It's essentially, either the sensor is not working, or its signal is not getting to the control board, or the board is not reliably responding. I suppose it could be a sticky gas control valve. What does the troubleshooting section say? You may need to get a service manual, and that may be hard if there's not much in the owner's manual.