- Heat pumps
- Air-to-water heat pump
UH18
Daikin Altherma 4 H F UH-18: Outdoor unit locked (R290)
Risk level
High
Professional inspection is recommended.
Urgency
Immediate
Quick answer
UH-18 means “Outdoor unit locked (R290)” on the scoped Altherma 4 H F family. Daikin records this trigger: Outdoor unit locked state. Effect: Heat pump will stop operating. 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
- Treat the sealed R290 system as professional-only
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for UH-18
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 UH-18
What they doGive the technician UH-18, both model numbers, the source trigger “Outdoor unit locked state”, the effect “Heat pump will stop operating”, reset result and recurrence conditions. They should follow Daikin's R290 refrigerant and compressor 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 UH-18 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
Use the displayed reset path once
No special tools
Procedure
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
- 3
Treat the sealed R290 system as professional-only
No special tools
Procedure
Keep flames and ignition sources away and do not loosen panels, valves or pipework. An R290, pressure, compressor or refrigerant-circuit decision requires a certified technician and the manufacturer procedure.
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 UH-18
- code erreur Daikin Altherma UH-18
- codice UH-18 pompa di calore Daikin
- código UH-18 bomba de calor Daikin
- Daikin Altherma error UH-18
- Daikin Altherma Fehler UH-18
- Daikin heat pump code UH-18
- Daikin UH-18
- Daikin Wärmepumpe Fehlercode UH-18
- défaut UH-18 pompe à chaleur Daikin
- error Daikin Altherma UH-18
- errore Daikin Altherma UH-18
What it means
In guide 4P773391-1C (2026.02), Daikin assigns UH-18 to “Outdoor unit locked (R290)”. The documented trigger is “Outdoor unit locked state” 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
- Criticalsource 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 “Outdoor unit locked (R290)” and gives this exact trigger: Outdoor unit locked state.
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: Manual. A cleared display is not proof of repair.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does UH18 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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