The door interlock is commonly known as a door lock. Understanding how a door lock works helps in deciding if one is faulty or not. When you shut the door, the latch on the door pushes inside the door lock and activates an electrical part inside which locks the door shut. At the same time, power is passed through the door lock on to the rest of the machine. The actual latching shut of the door is a simple spring-loaded mechanism where the latch just slips into the door catch and the door stays closed. Locking the door is a separate process.
How do washing machines lock the door?
Locking the door so it can’t be opened whilst the washer is running is usually achieved using a bi-metal part inside the door lock. I’ve been repairing washing machines for over 30 years and seen many different types of lock some of which do not use a bi-metal device at all. Some (e.g. some old Hotpoint washing machines) had a more sophisticated system that only locked the door when the motor was running and/or there was water inside. It achieved this using a pneumatic device (discussed in part 2). However, the majority of washing machines just use a bi-metal strip inside the door lock, which is cheap and effective.
How does a bi-metal device work?
Two different types of metal with different properties are combined on top of each other. When the door is closed, a connection inside the lock powers an electrical coil wrapped round the bi metal part. This heats up the metal, which then bends. This bending operates the lock and then makes another electrical connection passing power to the rest of the machine. Once power is turned off, the metal strip cools down after a minute or two and straightens back up again releasing the door.
How the door lock passes power on
In this rudimentary diagram you can see how the description above looks. The red line represents the bi-metal strip and the zig zagged lines between the live and neutral are the coil. Power activates the coil, which heats up the bi-metal part, this then physically bends and pushes the switch to the on position (as well as mechanically locking the door shut). Once this has happened (usually a second or so) power can run around to the common wire at the end, and therefore on to the rest of the machine allowing it to start.
Note: Do not assume anything about the wire connections. They may be in totally different positions on different machines. Unless the connections are marked Live Neutral and Common you would need to work it all out with a continuity test meter. If you get the wires the wrong way round you can blow the device. You may even blow the main PCB. It may be possible for a fourth wire on some variants to carry power to an LED which may indicate that the door is closed and the lock activated.
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How can you tell if a door lock is faulty?
There isn’t a simple test, although you may be able to get a resistance reading between the live and neutral which should measure the small coil. You might expect to get a resistance reading of something like 1000 Ohm. As with most fault diagnosis it’s mostly a matter of the process of elimination, observation and previous experience. But as my opening paragraph states, if you understand exactly how something works you are much more likely to be able to diagnose why it’s not working.
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Faults to look out for
First, the obvious. Are there any connection faults on it? Wires can come loose, or overheat and burn. A lack of power, neutral return, an open circuit coil, or even the door latch not properly activating an otherwise perfectly functioning door lock can all cause faults.
The coil can go open circuit or it can develop mechanical faults preventing its proper operation. Poor connections or burned wires are common. If wires are burned and have gone discoloured or hard they need stripping back to good wire and new connectors fitting. Also replace the lock because it will have overheated connections inside too.
In order to operate, the door lock needs the door catch to click firmly in place. If the door catch has broken, come loose, or the door isn’t closing properly it won’t operate the lock. It’s always worth spraying the inside of the door lock and door catch with WD40, it costs nothing and you never know.
Anomalies to be aware of
Historically virtually all UK washing machines have used this system. However it seems that some washing machines have now involved a relay built into the PCB that can cause the door lock to click but not lock. See this thread – Bosch WVG30461GB/01 won’t lock and start
After the door lock has been activated and the door locks shut, other faults could stop the washing machine from doing anything. For example, (This is a possibility but much more rare) if the common wire at the back of the door lock has a break in it somewhere or faulty connection where it connects to other parts of the machine so the power passing through the lock cannot reach its intended destination . The door may well lock, but this only shows that the lock is working. It doesn’t prove that the connection inside the lock is passing power on, nor does it prove that the wire used to pass power on is connected properly…
A fault inside the main PCB, or even an open circuit motor or heating element on some washing machines can result (bizarrely) in the door lock activating and locking with displays and lights coming on but washer will not do anything at all. Some Hotpoint and Indesit machines have this anomaly.
Different types of door lock
The most common type by far is the simple bi metal device described above, usually with three wires (see photo). They differ in size, shape and colour – see Ransom Spares Door locks section so see what they look like (or buy). The three wires are normally, the main live supply, a neutral return for the coil which heats up the bi metal strip, and the third wire is used to carry the electricity from the door interlock once it’s been activated across to the rest of the machine.
Buy a new door lock
Ransom Spares door parts Door locks section
Be Safe: Don’t risk it
If you aren’t experienced and trained for working on electrical and mechanical appliances you should probably book an appliance engineer.
Even if experienced, please take a moment to check out my DIY repair safety advice, which not only covers electrical safety but has tips and advice that you may be less aware of too. Don’t be complacent.
Don’t get carried away
Over the years, even experienced appliance repairmen have been seriously injured or killed repairing appliances.
Read Part 2
NOTE: Not all door locks are the same. This subject continues on another page where it looks at Different types of door locks
Riccardo says
Thanks for that Andy – I had always assumed the door lock was far more complicated than this, but have never had one go seriously wrong, so had never bothered to take one apart to find out. My misunderstanding is now de-bunked so if I ever do have one apparently faulty, I will know what to look out for!
Whitegoodshelp (Andy Trigg) says
Thanks Ricardo. Some of them can be more complex but the vast majority are these simple bi-metal devices. My second part looks a little at variations on the door interlock
Louis Milichamp says
Hi Andy,
Am currentley having a lock issues with an Indesit machine (iwde 126).
Couple of months ago, It burnt out the cables at the rear of the lock and the lock mechinism was replaced and wires striped back and good connections made.
Yesterday the cables burnt again and though this time the lock was ok, so stripped back cables and reconnected.
Though now for some reason now the lock section works fine but any washing program the machine just fills and then unloads water constantly, but all the drying programs all work fine?
Has the lock issue damaged the pcb do you think? Or even damaged a sensor?
Tracey says
My washing machine door wont lock not sure if its the catch how do i check it
Andy Trigg says
Hi Tracey, the door catch shouldn’t be broken or loose. They normally have a spring on them, which forces the door catch in one direction, but allows the catch to be moved in the opposite direction against the spring. If the door catch feels strong and you can push it back against the spring which then returns to position when you let go and it isn’t broken, or the door hasn’t dropped so that it’s totally out of line, then it should be activating the interlock when the door is closed. If it appears to be okay then the fault may be on the door interlock, but as my article explains, there are other possible causes. If, after checking the door catch and reading my article you cannot see anything obvious it’s best to call in an engineer.
Mike says
Hi Andy, having problem with indesit washing machine, one time we used the wash cycle was working well but as the cycle got to the end the machine stopped the cycle and the lights on the front all started to flash and error code appeared on the screen f06, now the machine door is locked and wash cycles won’t start, any feedback very grateful
rosie says
i have a hotpoint wmf720 i shut the door and it lights up door meaning not shut . help
Andy Trigg says
Mike, F06 is listed as “not used” on my Indesit error code lists. Will the door open with the machine switched off or unplugged?
Andy Trigg says
Hello Rosie, that’s one of the problems this article is designed to help with. All I can say is that all of my thoughts and advice are within the article. It explains how the door interlock works and how it recognises when the door is shut as well as which parts are responsible for activating it. All of this is necessary in order to try and diagnose a fault. Unfortunately it’s not a case of being able to say that if the door closes but the light lights up saying the door is not closed it must be caused by one fault. It could be caused by various things which need diagnosing by process of elimination.
Helen says
Hi Andy got a Hotpoint Aquarius + WMF760 washing machine that started the wash process okay but then I noticed it was making a clicking noise and when I went to investigate, all lights were flashing on the front – one said “door”. I have turned it off and on with no luck and then left it off for a while to pull itself together but sadly to no avail so now I have clothes trapped in my machine. When it was flashing it was also making a clicking and the kitchen lights were flickering along with it.
Any advise would be wonderful?
Many thanks