- Boilers
- Electric water heater
W23
Thermor electric water heater W23: water sensor control fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
W23 (also shown as W.23, Erreur 23, Err3 or Err03, or 3 short flashes of the internal red LED every 3 seconds on Duralis) signals a water temperature control fault and stops heating immediately for safety. A technician must inspect the sensor probe and its pocket.
Safety first
Stop and check this first
- Do not open the electrical cover or attempt to pull out, bypass or bridge the water temperature sensor yourself: W23 / Err 03 shuts down heating to prevent dangerous water overheating, and live 230 V wiring is exposed inside the cover.
- If you see any water dripping or rust staining around the bottom cover of the water heater, switch off its dedicated circuit breaker at the electrical panel immediately while awaiting service.
Safe checks you can perform
- Record the exact code on the screen or the outer indicator flash pattern
- Read the measured water temperature in the digital information menu
- Check externally for water drips or rust at the bottom cover and isolate power if wet
Leave these tasks to a qualified professional
- Do not attempt this professional task: Withdraw the regulation probe, measure its ohmic value and inspect the sensor pocket
- Do not attempt this professional task: Replace both the heating element body and probe or thermostat if the pocket shows moisture or rust
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.
Withdraw the regulation probe, measure its ohmic value and inspect the sensor pocket
What they doAfter isolating mains power at the breaker and removing the technical cover, the technician must slide the temperature probe out of its pocket (doigt de gant), check that both probe and tube are completely dry and undamaged, and measure the probe's ohmic resistance against Thermor's chart.
They must show youthe physical condition of the removed temperature probe, the dry interior of the sensor pocket and the multimeter resistance reading.
In writingthe measured ohmic resistance of the probe, the condition of the sensor pocket and the part number of the replacement probe or electronic thermostat assembly.
Replace both the heating element body and probe or thermostat if the pocket shows moisture or rust
What they doIf withdrawing the probe reveals water, limescale or rust inside the watertight sensor pocket (doigt de gant), the professional must drain the tank and replace both the heating element body (corps de chauffe) and the temperature probe or complete electronic thermostat assembly.
They must show youthe removed heating element body or sensor pocket showing moisture, scale or rust traces and the new flange gasket fitted.
In writingthe part numbers for the replacement heating element body, seal and temperature probe or thermostat assembly plus leak and heating test results.
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.
Record the exact code on the screen or the outer indicator flash pattern
- Who:
- you, from outside the appliance without removing any covers.
- How:
- note whether the display reads W23, W.23, Erreur 23, Err3 or Err03 with flashing water-drop or shower-head icons (on Duralis Connect, Malicio or Aéromax), 2 flashes of the heating element icon (on Airlis), or rapid flashing on the external light (green on Duralis, red on ACI Visualis).
- Result that matters:
- confirming that the water heater has locked out heating due to a temperature control fault and capturing the exact indication for the technician.
- If the fault code or rapid flash is active and water is cold:
- check the temperature reading in the digital menu if your heater has a screen.
- If the outer green light on a Duralis or Visualis heater flashes slowly once every 5 seconds and water is hot:
- that is normal ACI battery operation, not W23/Err03.
- Evidence to keep:
- photo or short video of the digital screen or outer indicator light showing the active fault.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
Read the measured water temperature in the digital information menu
- Who:
- you, using only the external front controls on models with a digital display.
- How:
- on water heaters equipped with a digital screen (such as Duralis Connect, Malicio or Aéromax), open the information or sensor status menu without opening the casing and check the reported water temperature.
- Result that matters:
- seeing whether the sensor reading is incoherent with actual tank temperature, which Thermor defines as below 10 to 15 °C or above 65 °C.
- If the screen displays an extreme reading (below 10 to 15 °C or above 65 °C):
- note the exact figure to share with the attending technician.
- If your water heater has no digital display (such as Duralis or ACI Visualis):
- do not open the cover to look for the internal red LED; move straight to the external leak check.
- Evidence to keep:
- photo of the temperature value shown in the menu together with the appliance rating plate.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
Check externally for water drips or rust at the bottom cover and isolate power if wet
- Who:
- you, by visual inspection from outside and at the household electrical panel.
- How:
- look underneath the water heater without dismantling the casing to see if water droplets, scale deposits or rust stains are coming from the lower cover seam; if you spot any moisture, switch off the water heater's circuit breaker on the electrical panel immediately.
- Result that matters:
- identifying a leaking sensor pocket (doigt de gant) or heating element flange early and preventing electrical hazards.
- If you see dripping water or rust traces beneath the cover:
- leave the circuit breaker off and tell the installer that both the heating element body and probe may need replacement.
- If the outside of the heater is completely dry:
- book a professional visit to test the temperature probe resistance and inspect inside the sensor pocket.
- Evidence to keep:
- photo of the underside of the water heater and the serial/model rating label.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
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
- Boilers
- Brand
- Thermor
- Product type
- Electric water heater
Known code variants
- Termo eléctrico Thermor W23
- Thermor Duralis 3 parpadeos rojos
- Thermor Duralis 3 red flashes
- Thermor Err 03
- Thermor Err03
- Thermor Erreur 23
- Thermor Erreur 3
- Thermor W.23
- Thermor W23
- Thermor water heater W23
What it means
Thermor's official page "Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ?" links W23/W.23/Erreur 23/Err3/Err03 to abnormal regulation probe readings received by the board or electronic thermostat on Duralis Connect, Malicio 1, 2 and 3, Duralis, ACI Visualis, Airlis and Aéromax. Duralis Connect/Malicio: Err03 with a flashing drop or shower-head icon; screenless Duralis: fast-flashing outer green LED and 3 short inner red flashes every 3 s; ACI Visualis: fast-flashing red LED. Heating stops to prevent overheating. Other series may use these codes differently. The code alone never proves a faulty part.
Warnings and stop conditions
- Higheditorial warning
Do not open the electrical cover or attempt to pull out, bypass or bridge the water temperature sensor yourself: W23 / Err 03 shuts down heating to prevent dangerous water overheating, and live 230 V wiring is exposed inside the cover.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
- Mediumeditorial warning
If you see any water dripping or rust staining around the bottom cover of the water heater, switch off its dedicated circuit breaker at the electrical panel immediately while awaiting service.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
Symptoms and causes
Probable causes · in order
- Regulation temperature sensor probe disconnected, out of calibration or open-circuitCommon
The water temperature probe sends an out-of-range ohmic resistance or an incoherent temperature value (below 10 to 15 °C or above 65 °C) to the electronic thermostat or control board.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
- Water seepage, moisture, limescale or rust inside the sensor pocket (doigt de gant)Possible
A breach or loss of watertightness in the heating element body allows moisture, scale or rust into the sensor pocket tube, corrupting the temperature probe reading.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
- Internal fault on the complete electronic thermostat assembly on Duralis modelsLess common
On Duralis electric water heaters where the probe is part of the electronic thermostat assembly, an internal module failure requires replacing the complete electronic thermostat assembly even when the pocket is dry.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ? · À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
Virtual technician
Does W23 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
Mon chauffe-eau affiche une erreur, que faire ?
Thermor · 2025 · official_doc
- 2
Comment résoudre un problème de chauffe-eau qui affiche une erreur 3, 23 ou W23 ?
Thermor · 2026 · official_doc
- 3
Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Thermor · 2026 · official_doc
- 4
À quoi correspondent les voyants sur mon chauffe-eau électrique ACI Visualis Thermor ?
Thermor · 2026 · official_doc
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