Daikin [H4] fault codewhat it means and how to fix it

H4 appears in Daikin’s Altherma error table as "Malfunction of low pressure switch", and that is the whole of what Daikin publish for owners. The low pressure switch is a safety device in the refrigerant circuit that opens if the suction pressure falls below a set point, and its purpose is to stop the compressor before it damages itself running on too little refrigerant. So H4 sits next to U0 in practice: one says the charge is short, the other says the device that guards against a short charge has either tripped or failed.

Daikin’s service documentation for the larger Altherma 3 H HT range gives H4-00 a troubleshooting page of its own and lists the low pressure switch among the outdoor unit’s repairable components, which tells you two things — that the switch itself is a replaceable part, and that the diagnosis is a job for whoever can open the outdoor unit. Nobody without F-Gas certification may do that on an R32 or R410A system. It is worth being honest about the limits here. Daikin’s owner-facing table gives a name, not a cause, and we have not read the detailed H4 procedure, so this page will not tell you whether your switch has tripped on genuinely low pressure, failed electrically or lost its connector.

An engineer with gauges will, in about ten minutes. What you can usefully establish is whether the unit still runs at all, whether the code appeared in very cold weather — the extreme of the operating envelope, where suction pressure is at its lowest — and whether anything else showed alongside it, particularly U0. Those three observations shorten the visit.