Mira [D] fault codewhat it means and how to fix it

Code D is the Advance catching an over-temperature at the point it matters most, the outlet, which is the last thing the water passes before it reaches you. The thermistor there is the sensor the shower uses to hold its temperature, so when it reports a reading above the safe limit the appliance has two pieces of information at once: the water is too hot, and its own control has failed to prevent that. Stopping is the only correct response.

What is interesting about this code is the remedy Mira publish for it, because it is not what you would expect. They do not send you to the thermostat or the electronics. They tell you to purge the flow valve three times for ten seconds, making sure cold water comes out of the showerhead each time. That points at what actually causes this in the field: the temperature at the outlet went up because the water slowed down, not because the shower decided to make more heat.

An electric shower puts fixed energy into moving water, so anything that restricts the flow — a scaled head, a partly blocked valve, debris holding the flow valve short of fully open — raises the outlet temperature directly. Purging the valve is Mira addressing the restriction rather than the symptom. The distinction from code F is worth holding on to, because the two look similar and mean opposite things: D is heat measured on the way out, which is usually the shower’s own flow. F is heat measured on the way in, which is not the shower’s fault at all.

1. 1

   Purge the flow valve exactly as Mira specify: three runs of ten seconds, confirming cold water actually reaches the showerhead on each one.

2. 2

   Take the showerhead off and check the spray plate. If it is furred, descale or replace it before you run the shower again.

3. 3

   Run the shower at a lower temperature setting and higher flow. If the head is clean and the code still comes back, the inlet filter is the next restriction, and on a Mira that is a tradesperson’s job behind the cover with a re-prime afterwards rather than something to do yourself.

   The outlet water can be genuinely hot after this fault, so let it run cold before you handle the head or hose. Do not remove the cover to reach the filter; Mira print that mains connections are exposed when it is off.
4. 4

   Run it for five minutes and watch the temperature rather than judging it on the first thirty seconds — Mira ask for five minutes in their own commissioning check.

5. 5

   If D comes back with a clean head and a purged valve, the thermistor or the control is at fault and the unit needs a service visit.