Has your Electric shower suddenly gone completely cold on you? 

Maybe you also heard a clicking noise as this happened? 

The most common reason for this is a failed Thermal cut out switch.

A Thermal Cut out switch (also known as a TCO) is an important safety component of an Electric shower. The function of this part is to cut the power to the heater tank if the temperature inside the unit gets too high. This is to protect the unit from overheating and the user from a sudden scalding.

An example of a common Thermal cut out switch (TCO) - Mira 439.90

In some instances, they can begin working again after the shower has been powered off and has cooled down, however this can be intermittent and they often fail entirely. To check if a TCO has failed, an electrician can test for continuity across the TCO terminals with a multimeter (with the shower isolated and made safe). The circuit will be open through the TCO if it has failed. If the TCO is 'good' the circuit will be closed and the multimeter will emit an audible tone.

We sell a great value Multimeter by Hayes as part of our Practical Supplies range, you can find this here.

Why did the TCO fail?

It's important to consider the root cause of why the TCO may have failed, before replacing the part . A failed TCO often points to another issue within the shower, such as blocked inlet filters, shower head or hose, or a scaled-up heater tank. These faults all cause the water flow within the heater tank to be reduced, leading to the water spending more time in the tank, and you guessed it, getting too hot!

This fault would need to be rectified before fitting a new one, otherwise it will most likely fail soon after replacing.


Note: If the shower is set on the highest setting through the summer or the temperature is turned up too quickly this can also have an impact. For more information see Seasonal effects on electric showers

Tip: When replacing a TCO, it's important that the wires are connected back exactly as they were removed. It can be useful to take a photo of the TCO's wiring position on the heater can before removing to refer back to later!

Take a look at this article which includes a video on some of the common problems with electric showers:  Video: Common problems with electric showers

For more information or to troubleshoot your shower further, why not take a look at our handy Fault Finder

Or for help identifying your Thermal Cut Out switch, or the shower itself take a look at our guide: Need help identifying your Shower or Part?