Range cooker U400 fault codewhat it means and how to fix it
A 400-family code means the cooker has been connected to the wrong supply or wired incorrectly. Five badges from four manufacturers publish a version of it, and it appears at installation.
Take care
Get a qualified appliance engineer to look at your range cooker
We do not pass enquiries to a repair firm, so there is no form here. When you book a qualified appliance engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
When did the code first appear?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
When did the code first appear?
Almost certainly an installation error rather than a faulty appliance. The code is the cooker checking its own supply and rejecting what it found.
Go back to whoever installed it before you contact the manufacturer. This is the installation, not the product.
When did the code first appear?
A disturbed or loosened connection is the likely explanation, and a loose connection on a 30 amp circuit is worth taking seriously rather than living with.
Isolate the cooker at the wall switch and get an electrician to check the connection before using it again.
When did the code first appear?
Less common, and it can point at a deteriorating connection or at something happening on the incoming supply rather than at the cooker.
An electrician should look at the cooker connection and the circuit. If they find nothing wrong, the supply itself is the next thing to check.
What U400 means
U400, and its close relative ER400, is the code that tells you a range cooker has been connected wrongly. It is unusual on this site in two ways: it appears at installation rather than after years of service, and four separate manufacturers publish a version of it under five badges with essentially the same meaning, which makes it one of the very few genuinely cross-brand codes in UK cooking appliances.
Smeg publish it as U400 and their entire remedy is six words long: wrong connection, connect the equipment properly. Rangemaster and Falcon, which share one troubleshooting document, publish U400 as the cooker having been incorrectly connected, with the control switching off after about a second and the code staying permanently displayed, and they are the only ones who add a line telling you who to call: consult your installer or a qualified repair engineer. Lamona publish U400 in the same words in a Howdens induction hob manual, with the same one-second switch-off and a continuous tone. Glen Dimplex publish the same fault as ER400 in their 2022-generation Stoves induction manual, and their remedy is the plainest of the lot — check the mains cable connection and that the terminals at the terminal block are wired correctly. Their 2025 tables have dropped ER400 altogether; the nearest thing in the 2025 Belling table is E3 or E4 for a supply voltage that may be too high or too low, which is a different fault with a different remedy. Different badges, different code strings, one fault.
What that means practically is that a brand new cooker showing a 400-family code has almost certainly been wired wrongly rather than delivered broken, and the person who needs to come back is the electrician rather than the manufacturer. A range cooker can be wired for several different supply configurations, and getting the links in the terminal block wrong, or connecting it to a supply it does not accept, produces exactly this.
It also turns up on an appliance that has been in service, and when it does the usual story is that something has been disturbed: the cooker pulled out for cleaning or for work behind it, a connection that has worked loose over years of heat cycling, or an electrician who has been in the consumer unit. In every one of those cases the message is the same and the fix is at the connection rather than inside the appliance.
One caution about the brevity of most of these remedies. Only Rangemaster and Falcon tell the owner to call anybody; Smeg, Lamona and Glen Dimplex simply say to connect it properly or check the terminal block, and none of that wording should be read as an invitation. On a hardwired range cooker on a 30 or 32 amp circuit, connecting the equipment properly is electrician’s work under Part P, not an evening job — and Smeg’s own installation section says the electrical connection must be made by authorised personnel.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Wired incorrectly at the terminal block during installation | By far the most common. These cookers accept several supply configurations and the links in the terminal block have to match the one you actually have. Glen Dimplex name the terminal block explicitly in their remedy. | Common |
| Connected to a supply the appliance does not accept | A different failure from a wiring mistake. The appliance has checked what it is being fed and rejected it, which is what the 400 family is for. | Common |
| A connection disturbed after the cooker was moved | The version that turns up on an appliance that has worked for years. Pulling a range cooker out to clean behind it puts strain on a connection that has been heat-cycling since the day it was fitted. | Occasional |
| Work done elsewhere on the circuit | A consumer unit change, a rewire, or a new circuit for something else can leave the cooker seeing something different from what it saw yesterday. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£150
Nothing if the installer comes back to correct their own work. An independent electrician attending to check and correct a cooker connection is the usual cost otherwise.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related range cooker fault codes
- How to isolate a range cookerUsually simple to fixAn electric or dual fuel range cooker has no plug: five manufacturers describe a wall switch near but not above the cooker on a 30 or 32 amp circuit. All-gas ranges are the exception.Related: supply and connection
- Oven will not turn onUsually simple to fixOn most range cookers the ovens will not heat until the clock has been set, and the timer has to be in manual mode. Three manufacturers publish this themselves.Related: same brand
- HUsually simple to fixH on a zone display means the glass above it is still hot enough to burn you. It is a warning rather than a fault, and every brand sets its threshold differently.Related: same brand
- Cooling fan runs on after cookingUsually simple to fixA range cooker fan that keeps running for up to an hour after cooking is designed behaviour, and the maker of Stoves and Belling tells you not to switch the cooker off to stop it.Related: same brand
- Gas oven goes out after a minuteGet it checkedA gas oven that lights and then shuts down within a minute is usually the flame supervision device, and three manufacturers publish the same sixty-second answer.Related: same brand
Related guides
Common questions about range cooker U400
The cooker is brand new. Should I send it back?
Speak to the installer first. Every manufacturer that publishes this code points at the connection rather than at the appliance, and a cooker rejected as faulty and then found to be miswired is a wasted week for everybody.
My code is ER400 rather than U400. Is that a different fault?
No. Glen Dimplex use ER400 on their older Stoves documentation for what Rangemaster, Falcon, Lamona and Smeg publish as U400, and the remedy they give is the same idea in plainer language. The prefix is a house style rather than a different diagnosis.
Can I just check the terminal block myself?
No. That means opening the connection point on a hardwired 30 or 32 amp circuit, which is notifiable electrical work in England and Wales and needs someone competent and registered. Isolating the cooker at the wall switch is yours; what is behind the terminal cover is not.
Why is Smeg’s advice so short?
Because that is genuinely all their support article publishes: wrong connection, connect the equipment properly. Lamona and Glen Dimplex are no more forthcoming, and only Rangemaster and Falcon add a line about calling anybody. We are reproducing each manufacturer’s wording rather than softening it or padding it out with advice they did not give — but Smeg’s installation instructions, in the same manual, do require the electrical connection to be made by authorised personnel, so their brevity is not permission.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.