- Heat pumps
- Air-to-water heat pump
U4-00
Daikin Altherma 4 H F U4-00: Indoor/outdoor unit communication problem
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
U4-00 means “Indoor/outdoor unit communication problem” on the scoped Altherma 4 H F family. Daikin records this trigger: Communication failure between outdoor and indoor unit. Effect: Heat pump will stop operating. The table marks the reset as automatic.
Safety first
Stop and check this first
- Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Safe checks you can perform
- Preserve the diagnostic state
- Let the automatic reset logic decide
- Record which connected function is unavailable
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for U4-00
What a technician must be able to justify
A technician is coming
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Measured handoff for U4-00
What they doGive the technician U4-00, both model numbers, the source trigger “Communication failure between outdoor and indoor unit”, the effect “Heat pump will stop operating”, reset result and recurrence conditions. They should follow Daikin's communication and connected component branch and compare live measurements with the service values before replacing parts.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Checks and evidence, step by step
Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
- 1
Preserve the diagnostic state
No special tools
Procedure
Photograph U4-00 and the complete display. Record both unit model numbers, operating mode, requested temperatures, weather, system pressure shown to the user and the moment the code appeared.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- 2
Let the automatic reset logic decide
No special tools
Procedure
Daikin marks this code as automatic reset. Do not cycle breakers or force repeated acknowledgements. Record whether the controller clears it and the exact operating state in which it returns.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- 3
Record which connected function is unavailable
No special tools
Procedure
Record which zone, pump, controller or outdoor function stopped and whether the problem followed a power event. Do not reseat internal plugs or change addresses, termination or field wiring.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Daikin
- Product type
- Air-to-water heat pump
Known code variants
- Altherma 4 H F U4-00
- code erreur Daikin Altherma U4-00
- codice U4-00 pompa di calore Daikin
- código U4-00 bomba de calor Daikin
- Daikin Altherma error U4-00
- Daikin Altherma Fehler U4-00
- Daikin heat pump code U4-00
- Daikin U4-00
- Daikin Wärmepumpe Fehlercode U4-00
- défaut U4-00 pompe à chaleur Daikin
- error Daikin Altherma U4-00
- errore Daikin Altherma U4-00
What it means
In guide 4P773391-1C (2026.02), Daikin assigns U4-00 to “Indoor/outdoor unit communication problem”. The documented trigger is “Communication failure between outdoor and indoor unit” and the operating effect is “Heat pump will stop operating”. This is a precise controller decision for Daikin Altherma 4 H F — EPSK(S)04~14A + EPVX07~14S18+23A; it does not by itself prove which component failed.
Warnings and stop conditions
- Highsource verified
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- What the controller establishedConfirmed
Daikin's table defines the condition as “Indoor/outdoor unit communication problem” and gives this exact trigger: Communication failure between outdoor and indoor unit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- What the unit does nextConfirmed
Documented effect: Heat pump will stop operating. Reset classification: Automatic. A cleared display is not proof of repair.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U4-00 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Daikin Europe N.V. · 2026 · official_doc
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