Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code E1

What E1 means on this model

The hot water sensor on the pipe leaving the heat exchanger has measured more than 185 degrees F and the unit has shut down. Bosch treats this as potentially serious and says not to assume the sensor is at fault, because scale on the heat exchanger or pre-heated water fed in from a recirculation loop will produce the same reading.

Symptoms you will notice

• The unit shows E1 and stops heating
• Scalding hot water at the tap
• Banging or knocking from the unit
• Hot water that was normal has become hotter over time

Common causes

The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:

• Scale build-up on the heat exchanger raising the outlet temperature
• Pre-heated water being fed into the unit
• A defective hot water temperature sensor
• A unit that has never been descaled

What you can safely check

These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.

• Ask whether the unit has ever been serviced. If not, the first action is a descale and heat exchanger removal to clean the combustion chamber, not a sensor replacement.
• Check whether a recirculation loop or another water heater is feeding pre-heated water into this unit.
• Verify the two red wires are tight and the sensor is clipped to the top, middle of the horizontal section of the hot water pipe.
• Check the sensor contacts for corrosion and clean them with light sandpaper or an eraser.
• Compare the sensor resistance against the temperature chart in the manual. A reading on the chart with a genuine overheat means the heater needs service, not a new sensor.