- Heat pumps
- Air-to-water heat pump
H9-00
Daikin Altherma 4 H F H9-00: OU: Malfunction of outdoor air thermistor
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
H9-00 means “OU: Malfunction of outdoor air thermistor” on the scoped Altherma 4 H F family. Daikin records this trigger: Outdoor air thermistor input is out of range. 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
- Do not substitute a sensor diagnosis
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for H9-00
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 H9-00
What they doGive the technician H9-00, both model numbers, the source trigger “Outdoor air thermistor input is out of range”, the effect “Heat pump will stop operating”, reset result and recurrence conditions. They should follow Daikin's sensor and measured value 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 H9-00 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
Let the automatic reset logic decide
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
Do not substitute a sensor diagnosis
Note the displayed temperatures and whether they are stable or implausible. Do not unplug, bridge, heat, cool or resistance-test any sensor and do not open either 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
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 H9-00
- code erreur Daikin Altherma H9-00
- codice H9-00 pompa di calore Daikin
- código H9-00 bomba de calor Daikin
- Daikin Altherma error H9-00
- Daikin Altherma Fehler H9-00
- Daikin H9-00
- Daikin heat pump code H9-00
- Daikin Wärmepumpe Fehlercode H9-00
- défaut H9-00 pompe à chaleur Daikin
- error Daikin Altherma H9-00
- errore Daikin Altherma H9-00
What it means
In guide 4P773391-1C (2026.02), Daikin assigns H9-00 to “OU: Malfunction of outdoor air thermistor”. The documented trigger is “Outdoor air thermistor input is out of range” 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 “OU: Malfunction of outdoor air thermistor” and gives this exact trigger: Outdoor air thermistor input is out of range.
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 H9-00 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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