Safety first: Turn OFF the breaker before opening any electrical compartments.

Check breaker: Verify the dedicated breaker for the water heater is ON. Reset if tripped. If it trips again immediately, do not reset repeatedly; call a professional.
Inspect junction box: Remove the electrical junction box cover on the heater. Confirm all wire nuts or terminal screws are tight and there are no burned wires.
Internal fuse: Many Bosch models have a small fuse on the control board. With power off, remove the front cover and visually inspect the fuse. Replace only with the exact same type and rating if blown.
Test outlet (for plug-in models): If your Bosch electric heater plugs into a receptacle, test the outlet with another device to confirm power.
Control board failure: If power is present at the unit but it remains dead, the main control board may be defective and should be replaced by a qualified technician.

Bosch tankless water heater does not start at all; no lights or display

Possible Causes