Smeg U400 fault codewhat it means and how to fix it
Smeg publish six words for this: wrong connection, connect the equipment properly. On a hardwired range cooker that means an electrician rather than anything you can do.
Take care
Get a qualified appliance engineer to look at your Smeg U400
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 this cooker last have someone working on its supply?
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 this cooker last have someone working on its supply?
This is the overwhelming majority of U400 sightings and it points at the installation rather than the appliance. A cooker that has never run is far more likely to be wired wrongly than to have arrived faulty.
Go back to whoever connected it, quote the code and the words Smeg publish for it, and do not accept a swap of the appliance as the first answer.
When did this cooker last have someone working on its supply?
Start with that work. A cooker that ran correctly for years and then reported a wrong connection is describing something that changed, and what changed was the connection.
Isolate the cooker and have the person who did the work check their own connection before anyone is asked to diagnose the appliance.
When did this cooker last have someone working on its supply?
Less common, and it usually means a connection that was always marginal has finally deteriorated, most often a conductor that was never properly clamped.
Have an electrician check the cooker outlet and the appliance terminal block. A connection deteriorating under load is worth finding for its own sake.
What U400 means
Smeg’s entire published text for U400 is "Wrong connection. Connect the equipment properly." There is no diagnosis to interpret and no remedy to follow, and the brevity is the most interesting thing about it. This is the sixth manufacturer in our own research to publish a code in the 400 family meaning the appliance has been connected incorrectly, and Smeg are the only one of them whose support article does not tell the owner to call anybody. That is not permission. On a range cooker wired into a dedicated cooker circuit, connecting the equipment properly is work on fixed wiring carrying tens of amps, and the instruction has an unnamed audience: the person who installed it. The article does not name them, but the manual does. Its electrical connection section is headed with a danger of electrocution notice and opens with "Have the electrical connection performed by authorised technical personnel", and it asks for an omnipolar circuit breaker near the appliance in an easily reachable position.
What the code actually reports is a mismatch between the supply the appliance has been given and the supply it expects. The Portofino manual publishes five ways the appliance can be connected — 220 to 240 volts single phase with neutral, two phase and three phase without neutral, and 380 to 415 volts two phase or three phase with neutral — each with its own cable, and it says the cable clamp must be changed for a two or three phase supply. Choosing the wrong one, or leaving the links for the wrong one in place, produces exactly this complaint.
So does a live and neutral crossed, a link left out, a cooker connected across two phases where three phase supply exists, or a connection that was made properly and has since been disturbed. The timing is the most useful diagnostic on this page. U400 characteristically appears immediately, on first use of a newly installed or newly reconnected appliance, rather than after years of service. A brand new cooker that has never worked and shows U400 is overwhelmingly likely to have been wired wrongly rather than delivered broken, which matters because it changes who you call and who pays. That is a conversation with the installer or the kitchen fitter, not a warranty claim against Smeg. There is a contradiction to flag. Smeg’s older support article lists U400 among the codes requiring an engineer visit, while the newer one gives the bare instruction with nobody named. Both are Smeg pages. In this instance the disagreement does not change the outcome much, because both readings end with a qualified person at the terminal block.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Connected in the wrong configuration at installation | The appliance can be wired more than one way depending on the circuit, and the fitting instructions specify which. The wrong choice reports as a wrong connection. | Common |
| A conductor in the wrong terminal | Live and neutral crossed, or a link left out of the terminal block, produces a supply the control does not recognise as valid. | Common |
| The cooker was disconnected and put back | Worktop replacement, flooring work or a kitchen move all involve someone reconnecting it, and the code often dates from that week rather than from the original fit. | Occasional |
| An incomplete connection at the cooker outlet | A conductor that has never been properly clamped can behave as though it is absent, so the appliance judges the connection wrong rather than merely loose. | Occasional |
| Connected across two phases | In a property with a three phase supply, taking the cooker circuit from two phases rather than one gives it a voltage it was never designed to see. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£300
Time on site
1h 30m
An electrician’s visit to check and remake the connection at the lower end, more if the cooker circuit itself needs altering. Nothing at all if the installer who made the connection comes back to correct their own work.
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 Smeg fault codes
- E5Get it checkedThe hob has judged the mains voltage signal invalid. Smeg say to check the supply voltage, which on a hardwired cooker is an electrician’s job rather than yours.Related: supply and connection
- SHOUsually simple to fixNot a fault at all. The oven is in showroom mode, so the control panel works and no heating is possible. Smeg publish how to switch it off.Related: same brand
- E2Get it checkedOverheating of the inductor on the hob, but E2 is also in Smeg’s pyrolytic oven list. Which display it is on changes the answer completely.Related: same brand
- Er03Usually simple to fixA knob or button is being read as held on. On these knob-operated range cookers that usually means a knob kept turned to A too long. Release it, then clean the hob as Smeg say.Related: same brand
- E6Get it checkedPower failure to the power board. Smeg publish a timed remedy rather than a flick of the switch: turn the appliance off for a full five minutes.Related: same brand
- EAGet it checkedA power circuit board error on the induction hob. Rangemaster publish an EA too, describing something different, so check which brand’s table you are reading.Related: same brand
Own a Smeg range cooker? Save it and its codes come first next time.
Related guides
Common questions about Smeg U400
Smeg do not say to call anyone. Can I sort it out myself?
The article’s sentence stops at "connect the equipment properly" and names nobody, which is unusual among manufacturers publishing a 400 family code. It is not an invitation. The manual is explicit where the article is not: the electrical connection section carries a danger of electrocution notice and says to have the electrical connection performed by authorised technical personnel. The connection in question is the fixed wiring feeding a high current appliance, and the person who connects it properly needs to be competent to work on that circuit.
It is brand new. Should Smeg replace it?
Ask for the wiring to be checked before anyone talks about replacing anything. This code is the appliance reporting on the supply it has been handed, and the commonest reason a new range cooker shows it is that the connection was made incorrectly an hour earlier. A replacement connected the same way behaves the same way.
Can I keep using it in the meantime?
No. The appliance has stopped because it does not recognise its supply as valid, and cycling the power to get one more meal out of it means running a high current appliance on a connection that has already been reported as wrong. Leave it isolated until somebody has looked at the terminal block.
Who pays?
That depends on who made the connection, which is why the timing question at the top of this page matters. Work done by an installer or kitchen fitter is theirs to put right, and an appliance engineer sent to a wiring fault is a chargeable visit with nothing to repair. Establish whether the last person at that terminal block was the installer before you book anyone.
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 support portal
- Manufacturer technical and service documentation
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.