Thermador Cooktop Error Code U400

The U400 error code on a Thermador cooktop indicates that the electronics have been connected to an incorrect voltage supply of 400V. The warning is triggered when the control board detects that the measured voltage on the power grid has reached 280V, and in some cases the alert can appear at 250V. This is a voltage mismatch fault, meaning the appliance is receiving power at a level outside its designed operating parameters. The cooktop's internal monitoring circuit continuously measures incoming supply voltage and activates this warning to protect sensitive electronic components from damage caused by overvoltage conditions.

The most common scenario that produces a U400 fault is improper installation wiring, where the cooktop is connected to a 400V three-phase supply or an incorrectly configured circuit rather than the intended single-phase supply voltage. In European and international markets, where both 230V single-phase and 400V three-phase services are common in residential settings, a wiring error at the terminal block or junction box can easily result in the cooktop receiving excessive voltage. This type of fault can also occur if a home's electrical service has been reconfigured or if the wrong circuit was used during installation.

Because the U400 is a warning rather than a hard lockout in all cases, the cooktop may still attempt to operate, but continued use under incorrect voltage poses a serious risk of permanent damage to the power electronics, inverter boards, and induction modules. The correct first step is to disconnect the unit from the power supply immediately. Once the voltage supply is corrected to the proper level, the warning will clear on its own without any manual reset procedure required.