Daikin [U4] fault codewhat it means and how to fix it

U4 is "Indoor/outdoor unit communication problem" in every Daikin Altherma table: the two halves of the system have stopped talking. It is worth separating from the more serious outdoor unit codes because Daikin’s own checklist starts with mundane things. The service manual for the Altherma 3 M gives the trigger as a communication failure between the outdoor and indoor unit, the effect as the unit stopping, and the reset as a power reset via the outdoor unit — not a button on the controller, and not automatic.

Then the checks, in order: the power supply, which Daikin say must be within 4 per cent of nominal voltage, with a power drop or a short circuit as the named causes; the wiring between the hydro PCB and the main PCB; the main PCB; the outdoor unit fan motor; and the hydro PCB. That list reads differently depending on what you own. On a monobloc such as the 3 M, both boards live in the outdoor unit and the "indoor unit" is the hydro module inside the same casing, so U4 is an internal wiring or board fault plus whatever the mains is doing. On a split — the Altherma LT with an ERLQ outside and a hydrobox inside, or the 3 H F — the two are joined by an interconnection cable that Daikin describe as the indoor unit’s main power supply as well as its communication line, and that cable runs through a wall and along the outside of the house, where rodents, garden work and weathered terminals reach it.

Two published details matter to an owner. Daikin note that if a U4 error occurs, room frost protection is not guaranteed, so an empty house in winter with U4 showing is a pipework risk rather than an inconvenience. And the code is not in the refrigerant circuit, which is where the expensive repairs live; a communications fault is usually wiring, a supply problem or a board. What you can usefully establish before the visit is whether both halves have power, whether anything has tripped at the consumer unit or been left off at an isolator, whether the lights have flickered or the supply has been disturbed, and whether any work has been done outside recently near where the cable runs.

1. 1

   Check both halves have power. There is usually a separate isolator for the outdoor unit, and it is easy for one to have been left off. On a split the hydrobox is normally fed from the outdoor unit, so an outdoor isolator left off darkens both.

2. 2

   Check the consumer unit for a tripped breaker or RCD affecting either supply, and think about whether the mains has been disturbed — flickering lights, a power cut, work on the street.

3. 3

   Think about what has happened recently — an electrician working, garden work, fencing, anything digging near where the cable runs outside.

4. 4

   Look at the outdoor unit for obvious damage to the cable where it enters, including chew marks at ground level.

   Look only. Do not open the electrical enclosure on either unit — that is an installer job and the supply is high current.
5. 5

   If the house will be empty in cold weather, do not leave it. Daikin publish that with a U4 error, room frost protection is not guaranteed.

6. 6

   Book your installer and tell them it is U4. Daikin’s published reset is a power reset at the outdoor unit, and their checklist starts with the supply and the wiring, so saying the code shortens the visit.