The Shower Doctor Surgery: How to replace a Mira TCO

If you find your hot water is intermittently cutting out, then you may have a problem with the thermal cutout component in your shower.

So, how can you test whether or not this is the case and replace the faulty piece?


As ever, make sure the shower you're working on is isolated from the electrical mains, not only by switching off the pull cord but also by shutting it off at the fuseboard, taking the necessary precautions to ensure nobody will reactivate it while you are doing your maintenance.

In terms of tools, you will need:

  • A screwdriver with Torx T10 bit or
  • A screwdriver with Torx T50 bit
  • A multi-meter
  • A camera

Which bit you need will depend on the age of your shower - you'll only know for certain which one you need once you remove the cover and assess the size of the screws you are dealing with.

Finally, if you are working above your shower area or your bath, make sure the plughole is covered, this is so you are protected against the possibility of losing any tiny screws that are likely to spring off when you start removing them as you replace your TCO.

How to test if a TCO is working

Of course, before replacing the TCO, you need to check this is actually the problem. To do this, it's important to understand how it works.

When the shower's heating tank overheats, the switch cuts out - it's meant as a precautionary measure in every model. However, if it cuts out too often or the temperature becomes too high it will fail completely, causing no power to pass from one end to the other.

Set your multi-meter to continuity and place it on either side of the TCO. If there is no continuity, then your thermal switch is faulty.

Removing the TCO

After removing the shower cover, the TCO in a Mira model sits on top of the heating tank. 

The screws to remove the component can be fairly difficult to take off, so you might have to slacked the heating tanks screws to give you a bit of room to manoeuvre here by pulling the tank forward to get better access.

Next, take a photo of the layout of your wiring. The way it is set out has changed over the years, so your design might well be different to other Mira showers - don't rely on guides you might find on the internet.

You should now release the earth screw, loosening the TCO itself. Before taking it out, slacken off the other screws holding wires in place, making it easier to separate it from the rest of the unit.

Replacing the TCO

Firstly, hold your new TCO in front of your old one, orientating it to check it will fit properly and helping you to work out how best to insert it. 

The easiest way to go about this is by disconnecting the wires one at a time, trying to keep them on the screw, before reattaching them to the new TCO - thus holding everything in place.

Be wary though, as the wires are relatively heavy and might have a tendency to spring away. It is for this reason that you have covered your plughole to avoid losing anything.

Slot the TCO back into place and sit it in top of the lever at the summit of the heating tank, before reattaching the earth wire.

Finally, you should go over all your screws and make sure everything is tightened up. It's also critically important to ensure everything is clear around the wires - they shouldn't be touching anything else. This can be quite tricky, but is a necessity.

Once you are happy you have achieved this, screw the tank back to how it was before and the job is done! 

For more information, check out our how-to video on YouTube.

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