- Heat pumps
- Air-to-water heat pump
AJ03
Daikin Altherma 4 H F AJ-03: DHW long heat-up abnormality
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
AJ-03 means “DHW long heat-up abnormality” on the scoped Altherma 4 H F family. Daikin records this trigger: Tank heat-up has been running abnormally long. Effect: Domestic hot water operation will stop. The table marks the reset as manual.
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
- Use the displayed reset path once
- Keep the check outside the casing
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for AJ-03
What a technician must be able to justify
Technician visit
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 AJ-03
What they doGive the technician AJ-03, both model numbers, the source trigger “Tank heat-up has been running abnormally long”, the effect “Domestic hot water operation will stop”, reset result and recurrence conditions. They should follow Daikin's exact controller decision 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.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Preserve the diagnostic state
Photograph AJ-03 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
Use the displayed reset path once
Daikin marks this code as manual reset. Only when the installation is safe, follow the reset instruction shown by the user interface or the exact user manual once. Do not enter maintenance mode; recurrence ends user troubleshooting.
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
Keep the check outside the casing
Use only normal controls and user-accessible observations. Do not remove covers, bypass a safety device or replace a part from the code alone.
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
Still not solved
If the error persists, stop here. See what the technician must do and show you.
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 AJ-03
- code erreur Daikin Altherma AJ-03
- codice AJ-03 pompa di calore Daikin
- código AJ-03 bomba de calor Daikin
- Daikin AJ-03
- Daikin Altherma error AJ-03
- Daikin Altherma Fehler AJ-03
- Daikin heat pump code AJ-03
- Daikin Wärmepumpe Fehlercode AJ-03
- défaut AJ-03 pompe à chaleur Daikin
- error Daikin Altherma AJ-03
- errore Daikin Altherma AJ-03
What it means
In guide 4P773391-1C (2026.02), Daikin assigns AJ-03 to “DHW long heat-up abnormality”. The documented trigger is “Tank heat-up has been running abnormally long” and the operating effect is “Domestic hot water operation will stop”. 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 “DHW long heat-up abnormality” and gives this exact trigger: Tank heat-up has been running abnormally long.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- What the unit does nextConfirmed
Documented effect: Domestic hot water operation will stop. Reset classification: Manual. A cleared display is not proof of repair.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does AJ03 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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