- Heat pumps
- Air-to-water heat pump
UJ20
Daikin Altherma 4 H F UJ-20: AF warning
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
UJ-20 means “AF warning” on the scoped Altherma 4 H F family. Daikin records this trigger: Active filter operation warning. Effect: Unit will continue operation. 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
- Isolate abnormal electrical symptoms
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for UJ-20
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 UJ-20
What they doGive the technician UJ-20, both model numbers, the source trigger “Active filter operation warning”, the effect “Unit will continue operation”, reset result and recurrence conditions. They should follow Daikin's power and electrical protection 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 UJ-20 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
Isolate abnormal electrical symptoms
No special tools
Procedure
Leave the system off after burning smell, heat, noise, sparking or repeated protective trips. Do not open electrical panels, test terminals or repeatedly re-energize the unit.
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 UJ-20
- code erreur Daikin Altherma UJ-20
- codice UJ-20 pompa di calore Daikin
- código UJ-20 bomba de calor Daikin
- Daikin Altherma error UJ-20
- Daikin Altherma Fehler UJ-20
- Daikin heat pump code UJ-20
- Daikin UJ-20
- Daikin Wärmepumpe Fehlercode UJ-20
- défaut UJ-20 pompe à chaleur Daikin
- error Daikin Altherma UJ-20
- errore Daikin Altherma UJ-20
What it means
In guide 4P773391-1C (2026.02), Daikin assigns UJ-20 to “AF warning”. The documented trigger is “Active filter operation warning” and the operating effect is “Unit will continue operation”. 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 “AF warning” and gives this exact trigger: Active filter operation warning.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- What the unit does nextConfirmed
Documented effect: Unit will continue operation. 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 UJ20 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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