Bosch GWH2400 ENG has an E2 message - Bosch AquaStar 2400ES-NG

Error Codes and Troubleshooting Bosch Tankless

In that case get it repaired under warranty. Do not attempt to repair it yourself for two reasons:

1. it could invalidate your warranty
2. unless you are GasSafe (or equivalent) registered, it's against the law to repair gas appliances, even your own, for obvious reasons, both explosion and carbon monoxide.

E2 is a cold water inlet temp sensor. Has your unit experienced any freezing conditions? If it senses below 36 degrees or above 90 degrees it will show this error.

If its normal temp, and you still get this error...check your sensor. Its an 8(kohm) sensor. Make sure its not open. If it reads ok and your temp is not an issue, then you probably have a bad control board

E2 is a cold water inlet temp sensor. Has your unit experienced any freezing conditions? If it senses below 36 degrees or above 90 degrees it will show this error.

If its normal temp, and you still get this error...check your sensor. Its an 8(kohm) sensor. Make sure its not open. If it reads ok and your temp is not an issue, then you probably have a bad control board

Try a new sensor . I cannot see who told you to wipe the pipe it will do nothing .

Before calling service technician, open following link for Bosch troubleshooting resources, including error codes, manual, Bosch contact info, and service bulletins.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Error codes are starting point for troubleshoot.
e2 is cold water temperature sensor fault.
Bosch error code says: 1) Check connector on wires coming from top of water valve for a secure connection.
2) Sensor may trip if water temperature drops below freezing to protect heater from freezing conditions Any damage due to freezing conditions is not covered under warranty.

Another contributing cause can be temperature thermostat set very high and tankless sensor is going bad, and very cold incoming water, or combination of factors above.

Also read manual for advisories and required maintenance. Some units require water filter be cleaned monthly.

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You posted under Rheem water heater
E2 sounds like a Bosch error code: E2: Cold water temperature sensor fault.
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Check wires coming from cold water line valve. Make sure incoming water is above freezing

Sounds like you have a scaled up heat x get it checked out by a bosch service agent

unplug power or turn off power,reboot, and try again