- Heat pumps
- Air-to-water heat pump
81-06
Daikin Altherma 4 H F 81-06: indoor R1T water sensor fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
Code 81-06 on a Daikin Altherma 4 H F means the indoor unit entering water temperature thermistor R1T (connected to board A1P) is malfunctioning, while the unit continues operating with automatic reset. Check [11] Malfunction history and [6.3] Sensors before calling your installer.
Safety first
Stop and check this first
- Do not open the indoor unit switch box to inspect PCB A1P yourself; Daikin requires a qualified technician to disconnect all power supplies for more than 10 minutes and verify less than 50 V DC before touching internal electrical components.
- Never bridge or disconnect thermistor R1T on board A1P to mask code 81-06; Daikin requires operating the unit with all thermistors and pressure sensors properly connected.
Safe checks you can perform
- Check the 81-06 error details and recurrence pattern in [11] Malfunction history
- Inspect the water temperature readings under [6.3] Sensors while the unit continues operating
- Note the installer contact in [6.2] Dealer information and mention wiring reference R1T (A1P)
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check the R1T connection on PCB A1P and measure thermistor R1T resistance
- Do not attempt this professional task: Verify R1T pipe seating and confirm automatic reset of 81-06 on the user interface
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.
Check the R1T connection on PCB A1P and measure thermistor R1T resistance
What they doAn authorised Daikin technician must isolate all power supplies, wait more than 10 minutes and verify less than 50 V DC inside the switch box, locate wiring diagram part number R1T on indoor PCB A1P, unplug the thermistor, and measure its resistance against the service manual table.
They must show youthe connector condition of R1T on board A1P and the multimeter ohm reading of thermistor R1T at the measured pipe temperature.
In writingmeasured R1T resistance in ohms, the corresponding water pipe temperature, and the condition of the R1T plug on PCB A1P.
Verify R1T pipe seating and confirm automatic reset of 81-06 on the user interface
What they doAfter reseating the R1T (A1P) connector or replacing an out-of-spec R1T probe, the technician must verify proper thermal contact on the indoor unit entering water pipe, restore power, and confirm that 81-06 clears automatically and accurate entering water temperature appears in [6.3] Sensors.
They must show youthe cleared home screen and the live entering water temperature reading in [6.3] Sensors matching an independent pipe thermometer.
In writingthe post-repair entering water temperature reading, confirmation that 81-06 cleared automatically, and any part number replaced.
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.
Check the 81-06 error details and recurrence pattern in [11] Malfunction history
- Who:
- The homeowner or system operator standing at the indoor user interface.
- How:
- Open the 81-06 banner on the home screen to read the long description, then set the user permission level to advanced end user and go to [11]: Malfunction history.
- Result that matters:
- Whether 81-06 is currently active or has cleared and returned automatically over multiple heating cycles.
- If [11] Malfunction history shows 81-06 remaining active continuously, that points to an open or shorted R1T thermistor or an unplugged R1T connector on board A1P.
- If 81-06 appears briefly and clears automatically, that suggests an intermittent contact at the R1T (A1P) connector or a thermistor drifting near a temperature boundary.
- Evidence to keep:
- A clear photo of the 81-06 message and the timestamped log in [11] Malfunction history.
Check the error history from the external user interface only; do not open the indoor unit switch box.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Inspect the water temperature readings under [6.3] Sensors while the unit continues operating
- Who:
- The homeowner or operator reviewing live status on the user interface.
- How:
- Go to [6.3]: Information > Sensors on the indoor controller and check the displayed water temperature readings while observing whether space heating and hot water continue functioning.
- Result that matters:
- Whether the entering water temperature reading is plausible and stable while the unit operates in fallback mode.
- If the entering water reading in [6.3] Sensors is implausible or fixed while the unit runs, that confirms the controller is detecting an abnormal signal on the R1T (A1P) channel.
- If the unit continues delivering heating and hot water with 81-06 present, that confirms the documented operating effect (Unit will continue operation) while awaiting a scheduled technician visit.
- Evidence to keep:
- A photo of the readings in [6.3] Sensors and notes on current heating and domestic hot water performance.
Do not attempt to access thermistor R1T or board A1P inside the indoor unit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Note the installer contact in [6.2] Dealer information and mention wiring reference R1T (A1P)
- Who:
- The homeowner preparing the service call.
- How:
- Open [6.2]: Information > Dealer information on the user interface and write down the installer contact details along with code 81-06 and the indoor unit model designation.
- Result that matters:
- The technician receives the exact error code and knows to inspect thermistor R1T on indoor PCB A1P.
- If you report 81-06 along with any recent indoor unit maintenance, that points to whether the R1T connector on A1P could have been disturbed.
- If no other error codes appear in [11] Malfunction history, that isolates the issue to the indoor entering water thermistor R1T circuit.
- Evidence to keep:
- Indoor unit model name, serial number, and the contact information from [6.2] Dealer information.
Leave the power supply switched on so automatic operation and freeze protection remain active until the technician arrives.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
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 81-06
- code erreur Daikin Altherma 81-06
- codice 81-06 pompa di calore Daikin
- código 81-06 bomba de calor Daikin
- Daikin 81-06
- Daikin Altherma error 81-06
- Daikin Altherma Fehler 81-06
- Daikin heat pump code 81-06
- Daikin Wärmepumpe Fehlercode 81-06
- défaut 81-06 pompe à chaleur Daikin
- error Daikin Altherma 81-06
- errore Daikin Altherma 81-06
What it means
In the Daikin Altherma 4 H F installer reference guide (4PEN773391-1C), code 81-06 is defined as Entering water temperature thermistor abnormality (indoor unit) when the entering water temperature thermistor inside the indoor unit is malfunctioning. The troubleshooting table explicitly directs technicians to wiring diagram part number R1T (A1P) for the connection of this thermistor, notes that the unit will continue operation, and marks the reset as automatic. The code alone does not prove the R1T thermistor probe is broken, because a loose R1T connector on PCB A1P, damaged internal wiring, poor pipe contact, or an A1P input circuit fault can trigger the same code.
Warnings and stop conditions
- Mediumsource verified
Do not open the indoor unit switch box to inspect PCB A1P yourself; Daikin requires a qualified technician to disconnect all power supplies for more than 10 minutes and verify less than 50 V DC before touching internal electrical components.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Mediumsource verified
Never bridge or disconnect thermistor R1T on board A1P to mask code 81-06; Daikin requires operating the unit with all thermistors and pressure sensors properly connected.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Resistance drift, open circuit, or short circuit in indoor entering water thermistor R1TCommon
Thermistor R1T, which measures the return water temperature entering the indoor unit hydronic module, has failed electrically or drifted outside the temperature-resistance range expected by board A1P.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose or oxidized R1T plug on indoor PCB A1P or damaged internal sensor leadCommon
Daikin's troubleshooting note points directly to wiring diagram part number R1T (A1P) for the connection of the thermistor, where a partially unseated plug, bent pin, or chafed wire can interrupt the signal.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Improper seating of thermistor R1T on the indoor unit entering water pipePossible
If the retaining clip or thermal contact of thermistor R1T on the indoor return pipe is disturbed during servicing, the temperature reported to PCB A1P can deviate abnormally.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Defective thermistor sensing input on indoor unit PCB A1PLess common
If thermistor R1T shows normal resistance across temperatures and the harness connection to A1P is intact, the analog sensing channel on PCB A1P itself can be faulty.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does 81-06 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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