The reading in two sentences
U400 on this oven means the supply voltage is incorrect and outside the normal operating range the appliance accepts — that is what the manufacturer published for it, and no more. Stop using it until the supply has been checked: running heating loads on a supply the control has rejected is what damages boards. One of the checks below is an owner check, and the rest of the work is behind a panel.
Error codes
Reference| Code | Meaning |
|---|---|
| U400 | The supply voltage is incorrect. It is outside the normal operating range. |
What else is happening when this shows
- Lights elsewhere dim as the appliance runs — That is the circuit telling you the problem is upstream of the appliance.
- The code appears at the moment a heavy load switches on — Heating and motor starts are when a marginal supply shows itself.
- NEAR MISS — nothing works at all and there is no display — That is a dead supply rather than a marginal one, and it is a different check: breaker, then outlet.
What triggers U400, most likely first
- A branch circuit carrying more than it should — It goes first because it is the commonest and it can be tested without tools: the appliance expects the supply arrangement in its instructions, and sharing that branch with another heavy load removes the margin the control requires.
- Too much on one branch circuit — The appliance is designed for the supply the manufacturer specifies on its own circuit, and sharing the branch with another heavy load turns a healthy supply into a marginal one.
- A control board that misreads the supply it is given — It is last because it is the least likely, and the only way to reach it honestly is to prove the supply itself is sound first.
The order to work through
- Ask what electrical work has been done recently — A disturbed connection is both the likeliest cause and the one with a safety edge to it.
- Find out what else shares the circuit — Switch the other loads off and run the appliance alone; if the code stops, the branch is the answer.
- Look and smell at the outlet — Discolouration, a warm faceplate or a scorch smell is a connection fault and it is an electrician job, not an appliance one.
The reset and the checks that follow it
- Reset the breaker once and watch it — Cut the breaker for a minute, restore it, and note whether the code comes back immediately or only under load.
About 5 min
- Switch off at the breaker for a minute, restore, and note whether the reading returns at once or only under load — Write down the answer: it decides whether an electrician measures at rest or while the appliance heats.
About 15 min
- Write down when the code appears — What was in use, the time of day and what else was running. That note is what a technician or an electrician works from.
About 5 min
- STOP HERE — supply measurement is licensed work — Do not open an outlet or a terminal block to check a voltage. That is where an owner check ends.
Before a panel comes off
Safety
Unplug the oven or switch off its breaker before any panel comes off — a capacitor can hold a charge after the power is off. The cavity and the glass stay hot long after the heat is off, so let everything cool first. On a gas model, if you smell gas, leave the building and call the gas utility from outside.
Where an owner check ends
Call as soon as the code survives a dedicated outlet and an otherwise idle circuit.
Other codes on a Bosch oven
What is deliberately not here
Treat this as the general Bosch oven reading. It stops short of anything model-specific — part numbers, harness colours, connector layouts and service-mode entry — because those differ by family and a technician takes them from the rating plate rather than from a code. A family- or model-specific page, where one exists, is linked from the code index below and is the closer answer.
Links: - [Bosch oven error codes](/blog/bosch-oven-error-codes)
Next steps
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
