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BOSCH U400

Bosch Cooktop Error U400: Supply Voltage Too Low

Clear meaning, realistic next steps, and safe guidance without turning the page into a long repair manual.

What it means

The cooktop has detected that the household supply voltage is below the minimum required for safe operation. The cooktop will not heat until adequate voltage is restored.

Severity

Medium - some user checks may help, but repeated faults often need service.

Can you fix it yourself?

Partial — start with DIY steps

Most likely cause

Household voltage drop during a high-demand period

Estimated time for safe first checks: 5–10 min for basic checks; Electrician visit if wiring fault confirmed.

Step-by-Step DIY Fix Guide

  1. SAFETY: Disconnect power at the circuit breaker before checking any wiring.
  2. Check whether other high-wattage appliances (washer, dryer, AC) are running simultaneously — try switching one off and retesting.
  3. Inspect the circuit breaker for the cooktop — check it is fully engaged (not tripped to middle position).
  4. Reset the breaker if needed: flip fully off, then fully on.
  5. If U400 persists after switching off other large loads, call a licensed electrician to check the supply voltage and wiring.

If the airflow problem comes back

  • If airflow still seems weak after the basic checks, the fan path or a deeper obstruction becomes more likely.
  • U400 persists after other appliances are switched off — indicates a supply or wiring problem needing an electrician
  • Circuit breaker trips repeatedly when the cooktop is in use

What This Error Means

Error U400 on your Bosch cooktop means: The cooktop has detected that the household supply voltage is below the minimum required for safe operation. The cooktop will not heat until adequate voltage is restored. The appliance's self-diagnostic system has flagged this condition and may have disabled the affected zone or function to prevent damage or unsafe operation.

The most frequent cause is household voltage drop during a high-demand period (multiple large appliances running simultaneously). Work through the causes and fix steps below in order — starting with the simplest check.

Some cases of U400 can be partially addressed by the homeowner. The steps below cover the DIY checks — if they do not resolve the condition, a technician is needed.

Induction and radiant cooktops draw significant power. Voltage issues are almost always a household electrical supply problem, not a cooktop fault. A licensed electrician can verify supply voltage and wiring capacity within an hour.

Most Likely Cause by Symptom

The Bosch cooktop may stop, pause, or refuse to complete the cycle normally.

Likely cause: Household voltage drop during a high-demand period (multiple large appliances running simultaneously)

Check first: SAFETY: Disconnect power at the circuit breaker before checking any wiring.

Common Causes

  • Household voltage drop during a high-demand period (multiple large appliances running simultaneously)
  • Loose, undersized, or corroded wiring connection at the circuit breaker or terminal block
  • Inadequate electrical supply to the home — utility undervoltage condition
  • Cooktop wired on a shared circuit rather than a dedicated 240V circuit

What Not to Do

  • Do not use an extension cord or adapter with a 240V cooktop — it is a fire risk and will cause undervoltage
  • Do not assume the cooktop is faulty — U400 is almost always a household electrical supply issue

Model and Display Variation Notes

Model-family notes

  • Bosch cooktop display wording and code formats can vary by series.
  • If your model behaves differently, check the owner manual before trying any deeper maintenance step.

Display and panel differences

  • Panel wording can vary by series, so confirm the exact code pattern before buying parts.

Parts, Tools and Service Options

Service option

Bosch service visit if the warning returns after the basic checks are complete.

Manual and model check

Check your exact model and manual before ordering any Bosch cooktop parts.

This section stays service-first because the page points more strongly toward support or professional repair than a routine parts purchase.

When Not to Keep Troubleshooting

U400 persists after other appliances are switched off — indicates a supply or wiring problem needing an electrician

  • Circuit breaker trips repeatedly when the cooktop is in use

How to Prevent It Recurring

  • Ensure the cooktop is on a dedicated 240V circuit — never share with other high-wattage appliances
  • Have an electrician check the terminal block connections when the cooktop is first installed or after any electrical work in the home

Related Error Codes

Extra notes

  • This page is based on Bosch support material and stays conservative where model-specific guidance may vary.
  • The goal is to help you identify safe first checks before you move into parts, service, or model-specific manual lookup.

Source and model notes

Last reviewed: 2026-04-07

Based on: Based on Bosch support material and edited into consumer-safe guidance for the exact code family on this page.

View Bosch US Official

Model coverage note: Bosch cooktop code meanings can vary by series, control panel, and model family, so use this page as a safe starting point rather than a replacement for the model-specific manual.

Important: FixThisError is an independent guide, not the manufacturer. Use your model-specific manual when the panel wording or behavior differs.

Always disconnect power before inspecting appliances. If unsure, contact a licensed appliance technician.