Mira C fault codewhat it means and how to fix it
Flashing red and yellow with Start/Stop off. A thermistor fault: the shower has lost the temperature sensing it relies on to stay safe.
Take care
Get a qualified electrician to look at your Mira C
We do not pass enquiries to a repair firm, so there is no form here. When you book a qualified electrician, 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.
What C means
A thermistor is a resistor whose resistance changes predictably with temperature, and it is how an electric shower knows anything at all about how hot the water is. Code C means the appliance can no longer trust that reading, and the shower stops rather than run without it — which is the right behaviour and worth understanding, because it explains why this is not a fault you can work around. Everything the Advance does to protect you depends on knowing the outlet temperature: modulating the elements, catching an over-temperature, deciding whether to allow the shower to start. Take the sensor away and the appliance is a heating element with no feedback, so it declines to run.
Mira publish two causes for it in order, and the order is a diagnostic in itself. The first is the connection rather than the sensor: their instruction is to check the thermistor connection at CN3 on the relay board. Connectors on a component that lives in a warm, damp enclosure work loose and corrode, and a disconnected sensor reads as a failed one.
The second is the sensor itself, failed in the heater tank. Both sit behind the cover, on a unit fed by a 40 amp circuit in a bathroom, which is why this page carries no steps: Mira’s own second instruction on this code is to contact customer services, and the useful thing an owner can do is describe the indication accurately rather than open the case. Knowing which of the two it is changes the cost significantly, and it is a five-minute check for somebody with the cover off safely.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Thermistor connector loose or corroded at CN3 | Mira name the connector specifically. A sensor that is disconnected reports identically to one that has failed, and this is the cheap outcome. | Common |
| Thermistor failed in the heater tank | Mira’s second published cause. The sensor lives in a hot, wet, scaling environment and it is a wear item over a long enough period. | Common |
| Cable damage between sensor and board | Chafing or heat damage to the loom inside the case, which presents the same way as a failed sensor from outside. | Occasional |
| Relay board input failed | The board itself unable to read a healthy sensor. Rarer and more expensive, and only established by substitution. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£70–£200
Time on site
45 minutes
A reseated connector is a visit and little else. A failed thermistor is the part plus labour, and on an older shower that total is worth weighing against a replacement unit.
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 Mira fault codes
- AGet it checkedFlashing red on the Status light. Mira group three water-supply problems under this one code: no supply, low pressure, or a blockage somewhere in the outlet.Related: same brand
- DGet it checkedFlashing red and yellow. The outlet thermistor has seen a temperature above what Mira allow, and the shower has stopped rather than deliver it.Related: same brand
- MUsually simple to fixFlashing white. Not a breakdown — the shower was switched off at the cord before it finished its shutdown, and it will not restart properly until it settles.Related: same brand
- LGet it checkedStatus light off entirely, with Start/Stop off. Mira call this an electrical fault and the action is to check power to the shower.Related: same brand
- FGet it checkedFlashing red and blue with Start/Stop flashing. The cold water arriving at the shower is already hotter than Mira allow it to be.Related: same brand
- IGet it checkedFlashing red and white with Start/Stop flashing. Mira call it a software error, and their published remedy is simply to power cycle the unit.Related: same brand
Own a Mira electric shower? Save it and its codes come first next time.
Related guides
Common questions about Mira C
Can I use the shower until it is fixed?
No, and it will not let you. The appliance stops precisely because it cannot verify the outlet temperature, which is the reading its over-temperature protection depends on.
Is a thermistor an expensive part?
The sensor itself is not. What decides the cost is whether it is the sensor or the connector, and on an out-of-warranty shower the labour to find out is the larger part of the bill.
Why does the fault code say to check a connector I cannot reach?
Because Mira write these tables for installers as well as owners. The CN3 instruction is aimed at whoever has the cover off, which is why the useful step for an owner is passing the code on rather than acting on it.
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 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.