Mira F fault codewhat it means and how to fix it
Flashing red and blue with Start/Stop flashing. The cold water arriving at the shower is already hotter than Mira allow it to be.
Take care
What F means
Code F is the one fault in Mira’s table that is not about the shower at all, and their published action says so in a single flat sentence: the cold water supply is above the maximum temperature. An electric shower is designed around cold mains. It adds a known amount of heat to water arriving at somewhere between about five and twenty degrees, and its whole safety case assumes the starting point is cold. When the inlet thermistor reports water arriving already warm, the appliance cannot guarantee what the outlet temperature will be, so it stops.
There are three ordinary reasons for warm water arriving at a cold inlet and none of them is a broken shower. The first is the plumbing: somewhere upstream the cold feed runs alongside or through a hot pipe, an airing cupboard or an unlagged run in a warm loft, and it picks up heat while standing still overnight. The second is the season, and it is more significant than people expect — mains water that is six degrees in February can be twenty in August, and in a long exposed run inside a hot building it arrives warmer still. The third is a genuine plumbing fault: a failed non-return valve or a crossed connection letting hot into the cold side, which is worth ruling out because it affects more than the shower.
What makes F worth understanding rather than just clearing is that the shower is right. Running it anyway, if you could, would deliver water hotter than the appliance is allowed to produce, which is exactly the scenario the inlet sensor exists to prevent.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Cold feed picking up heat before the shower | Pipe running through an airing cupboard, alongside a hot pipe or through a warm loft. Worst first thing, when the water has stood in it all night. | Common |
| Seasonal rise in incoming mains temperature | Summer mains can be well over ten degrees warmer than winter mains. In a long run inside a warm building the inlet can cross Mira’s limit. | Common |
| Hot water crossing into the cold supply | A failed non-return valve or a crossed connection on a mixer elsewhere. This one shows up at other cold taps too, which is how you spot it. | Occasional |
| Inlet thermistor reading high | The sensor itself drifting. Considered only after the supply has been checked, because a genuinely warm inlet is far more likely. | Rare |
What to try yourself
Run the nearest cold tap for a minute and feel the water. If it comes through warm and then cools, the cold feed is picking up heat somewhere upstream.
Run the shower cold for a minute or two to draw fresh mains through, then try again. Overnight standing in a warm pipe is the commonest version of this.
Check other cold taps around the house. Warm water at several of them points at hot crossing into the cold side rather than at one badly routed pipe.
If the pipe feeding the shower runs through an airing cupboard or a warm loft, having it lagged or rerouted is the permanent fix.
If the cold supply is genuinely cold everywhere and F persists, the inlet sensor is suspect and the unit needs looking at.
Get a qualified electrician to look at your Mira F
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 it costs to fix
Typical repair cost
£0–£200
Time on site
30 minutes
Nothing if it is seasonal or a standing pipe. Lagging or rerouting a cold feed, or tracing a crossed connection, is a plumber’s visit rather than a shower repair.
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
- 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: overheat
- 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
- 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
- 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
- CGet it checkedFlashing red and yellow with Start/Stop off. A thermistor fault: the shower has lost the temperature sensing it relies on to stay safe.Related: same brand
Own a Mira electric shower? Save it and its codes come first next time.
Related guides
Common questions about Mira F
Is code F a fault with my shower?
No, and that is the useful thing about it. Mira’s own action for F describes the water supply rather than the appliance: the cold water arriving is above the maximum temperature the shower is designed for.
Why does it only happen in the morning?
Because the water in the pipe has been standing in a warm space all night. The first draw comes through warm and then the temperature drops as fresh mains arrives, which is why running it for a minute often clears it.
Could this happen because of a fault elsewhere in the plumbing?
Yes. Hot water crossing into the cold side through a failed non-return valve or a crossed mixer will do it, and that is worth finding, because it affects every cold outlet rather than only the shower.
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.