How to test an NTC thermistor

What Happens When the Door Is Closed

Closing the door is a two-stage process. First, the mechanical latch on the door clicks into the door catch – this is a simple spring-loaded mechanism that holds the door physically closed. Second, and separately, the door lock electrically locks the machine to prevent the door from being opened during operation.

When the door latch pushes into the lock body, it activates an electrical component inside. This component locks the door and simultaneously passes power on to the rest of the machine, allowing it to start. These are two distinct actions – mechanical latching and electrical locking – and a fault in either can prevent the machine from running.

How Do Washing Machines Lock the Door?

The majority of washing machines lock the door using a bi-metal device inside the door lock. This is an inexpensive and reliable solution used across most mainstream brands. Some machines – particularly older Hotpoint models – used more sophisticated systems that locked the door only when the motor was running or water was present. These are covered in the companion guide: different types of door interlock.

How Does a Bi-Metal Device Work?

A bi-metal device consists of two different metals bonded together, each with different expansion properties. When the door is closed, an electrical coil inside the lock is energised. The coil heats up the bi-metal strip, causing it to bend. This bending physically operates the door lock mechanism and simultaneously makes a new electrical connection, allowing power to pass on to the rest of the machine.

When the cycle ends and power is removed from the coil, the bi-metal strip cools – which takes one to two minutes – and straightens back, releasing the lock. This is why there is a short wait before the door can be opened after a cycle.

How the Door Lock Passes Power On

A standard bi-metal door lock has three wires. The first carries the main live supply that powers the coil. The second is the neutral return for the coil circuit. The third – called the common wire – carries power from the lock to the rest of the machine once the lock has activated. This sequence ensures the machine cannot start until the door is confirmed as locked.

Wire connections vary by machine – never assume.

The position and labelling of Live, Neutral, and Common wires differs between models. Unless the terminals are clearly marked, connections must be confirmed with a continuity test meter before disconnecting. Fitting a replacement with the wires in the wrong positions can destroy the lock and damage the main PCB.

Some variants have a fourth wire carrying power to an LED indicator that shows the door is closed and the lock is activated. This is additional to the three core wires described above.

How to Test if a Door Lock Is Faulty

There is no single definitive test for a door lock, but a resistance reading between the live and neutral terminals can indicate whether the coil is intact. A working bi-metal coil typically reads around 1000 ohms, though this varies by model. An open circuit reading (no continuity) suggests the coil has failed.

Beyond electrical testing, fault diagnosis relies on systematic elimination – checking wiring, connections, and the mechanical operation of the door catch before concluding the lock itself needs replacing.

Common Faults to Check

Before replacing a door lock, check the following:

• Wiring and connections. Wires can come loose over time or overheat and burn. If any wiring near the lock is discoloured, hard, or brittle, it has overheated. Strip back to undamaged wire and fit new connectors – and replace the lock too, as the overheated connections inside will have been compromised.
• The door catch. The lock cannot activate unless the door catch clicks firmly into place. A broken, loose, or misaligned door catch will prevent the lock from operating, even if the lock itself is in perfect condition. It is worth applying a small amount of WD40 to the catch and the inside of the lock housing – this costs nothing and can free a stiff mechanism.
• Power supply to the lock. A lack of live feed or neutral return will prevent the coil from activating, meaning the lock will not engage.
• The common wire beyond the lock. If the door locks but the machine does nothing, the common wire running from the lock to the rest of the machine may have a break or poor connection. The lock activating proves the lock is working – it does not confirm the common connection is sound.

Anomalies That Can Be Confused With Door Lock Faults

On some machines, the door may lock and displays may come on, but the machine refuses to do anything at all. This can be caused by a fault in the main PCB or – on some Hotpoint and Indesit models – by an open circuit motor or heating element. In these cases the door lock is functioning correctly, but a fault elsewhere is preventing the machine from proceeding. Replacing the door lock in these situations will not resolve anything.

Some machines also have a relay built into the PCB that can cause the door lock to click without actually locking – a fault that mimics a door lock problem but originates in the PCB. See the Washerhelp forum for a documented example: Bosch WVG30461GB/01 won’t lock and start.

Safety Warning

Working on electrical appliances carries serious risks.

Over many years, even experienced appliance engineers have been seriously injured or killed working on domestic appliances. Always disconnect from the mains before working on any internal component. If in any doubt about competence or safety, book a qualified appliance engineer.

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