Thermador Cooktop Error Code U400

The U400 code means the cooktop has detected that it is connected to an incorrect voltage supply, specifically a 400V circuit. The warning activates when the measured grid voltage reaches 280V, and sometimes triggers at 250V. It is a protective alert designed to prevent damage to the cooktop's electronics from overvoltage. This is an installation or wiring issue, not an internal component failure.

A simple reset will not permanently clear U400 because the error is caused by the voltage condition on the incoming power supply, not a software glitch. The warning will reappear as soon as the cooktop is powered on if the underlying voltage problem has not been corrected. The error clears automatically once the cooktop is connected to the correct supply voltage.

If the issue is a wiring or circuit error, the primary cost is an electrician's service call and any rewiring labor, which can range from around $100 to $400 or more depending on the complexity of the work needed. If the control board's voltage sensing circuit has been damaged due to prolonged overvoltage exposure and requires replacement, parts and labor costs for a Thermador control board repair can range from $300 to $700 or higher.

Start by disconnecting the cooktop from the power supply immediately. Have a licensed electrician measure the voltage at the connection point and verify it matches the cooktop's specified supply voltage. Once the incorrect voltage source is corrected and the cooktop is reconnected to the proper circuit, the U400 warning will clear on its own.

No, you should not continue operating the cooktop while U400 is active. The cooktop is receiving voltage beyond its designed operating range, which poses a risk of permanent damage to the power electronics, inverter boards, and induction components. Disconnect the unit from power immediately and do not use it until the voltage supply has been corrected by a qualified electrician.