- Boilers
- Electric water heater
Err 01
Thermor electric water heater Err 01: ACI battery charge fault
Risk level
Low
Usually safe to check without specialist tools.
Urgency
Low
Quick answer
Err 01 (or 1 short flash of the internal red LED every 3 to 5 seconds paired with fast green flashing on the front light of Duralis units) signals an ACI anti-corrosion battery fault and requires 16 to 24 hours of power to recharge. It does not stop water heating.
Safety first
Stop and check this first
- Never remove the lower cover or open the thermostat casing yourself to view the internal red LED near the ACI plug or touch the battery: live 230 V terminals are inside and must only be accessed by a qualified professional after isolating mains power.
- On Duralis and ACI Visualis water heaters, a slow green flash on the outer light every 5 seconds during peak hours is normal operation showing that the ACI anti-corrosion system is running on battery backup.
Safe checks you can perform
- Check the outer indicator light and confirm hot water is still available
- Allow two full off-peak cycles to recharge a new or recently turned-off unit
- Check Err 01 after recharging with the power supply restored
Leave these tasks to a qualified professional
- Do not attempt this professional task: Inspect the off-peak contactor circuit to guarantee at least 8 hours of daily supply
- Do not attempt this professional task: Verify 230 V supply and replace the complete electronic thermostat assembly
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.
Inspect the off-peak contactor circuit to guarantee at least 8 hours of daily supply
What they doWhen the battery only loses charge after returning to automatic off-peak mode, have an electrician test the day/night contactor on your electrical panel so the heater receives a minimum of 8 hours of power every day.
They must show youthe voltage test across the off-peak contactor output and proper switching between automatic and forced positions.
In writingthe electrical inspection report for the off-peak control circuit and the daily powered hours verified.
Verify 230 V supply and replace the complete electronic thermostat assembly
What they doIf the water heater is not new and still shows Err 01 or 1 internal red flash after 16 to 24 hours of verified power supply, have a qualified technician check the 230 V terminals and replace the complete electronic thermostat assembly.
They must show youthe internal red LED next to the ACI connector flashing once every 3 to 5 seconds before replacement and normal indicator operation afterwards.
In writingthe part reference of the replacement Thermor electronic thermostat assembly, measured supply voltage and confirmation of ACI protection.
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 outer indicator light and confirm hot water is still available
- Who:
- you, without removing the bottom cover or opening the electrical casing.
- How:
- inspect the external indicator light on the front of the water heater (on Duralis it flashes green rapidly when a fault is active, whereas on ACI+ or ACI Pro the light goes out during peak hours) and open a hot tap to verify that water heating still works.
- Result that matters:
- confirming that domestic hot water continues to heat, as Thermor states that Err 01 has no direct link with water heating unless mains power is missing.
- If hot water is available while the alert remains active:
- proceed to give the unit a 16 to 24-hour battery recharge cycle.
- If there is no hot water at the taps either:
- check your electrical panel right away for a tripped breaker or an off-peak contactor stuck in the off position.
- Evidence to keep:
- short video of the outer indicator light cadence and a note confirming whether hot water flows.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Allow two full off-peak cycles to recharge a new or recently turned-off unit
- Who:
- you.
- How:
- if the electric water heater has just been installed or you switched off power during a trip, leave the electrical supply connected through at least two 8-hour off-peak periods (16 cumulative hours of power) without turning the breaker off again.
- Result that matters:
- allowing the ACI anti-corrosion accumulator to reach full charge so Err 01 or the rapid green flashing clears by itself.
- If the indicator light returns to normal after 16 hours of power:
- the backup battery had simply discharged.
- If the alert remains active after two days on off-peak tariff:
- record the recharge time and whether Err 01 remains, as described in the next check.
- Evidence to keep:
- note of the date and time when continuous or off-peak power was restored to the water heater.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Check Err 01 after recharging with the power supply restored
- Who:
- you, observing the alert after the ACI battery recharge.
- How:
- once power is restored, leave the water heater powered: the battery recharges over two off-peak cycles of 8 hours with an off-peak contactor, or 16 hours on a 24/24 contract.
- Result that matters:
- checking whether Err 01 is gone after that recharge period.
- If Err 01 no longer appears after recharging:
- record that the alert has cleared.
- If Err 01 still appears at the end of recharging:
- note that the alert remains.
- Evidence to keep:
- date and time power was restored, the recharge cycles or hours elapsed, and whether Err 01 is present afterwards.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
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 Err 01
- Thermor ACI batería Err 01
- Thermor ACI battery fault Err 01
- Thermor Duralis 1 parpadeo rojo
- Thermor Duralis 1 red flash
- Thermor Err 01
- Thermor Erreur 01
- Thermor Erreur 1
- Thermor Visualis Err 01
- Thermor water heater Err 01
What it means
Thermor's official page "Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ?" links Err 01 or Erreur 01 to an electronic thermostat battery fault in ACI/ACI Hybride heaters (Duralis, ACI Visualis), powering tank corrosion protection in peak hours with an off-peak contactor. Duralis: fast green outer light and 1 short internal red LED flash every 3 to 5 s; ACI+/ACI Pro without an error LED: ACI light off in peak hours. Outside these ACI ranges, indicators may differ. This alert alone cannot establish a defective thermostat.
Warnings and stop conditions
- Higheditorial warning
Never remove the lower cover or open the thermostat casing yourself to view the internal red LED near the ACI plug or touch the battery: live 230 V terminals are inside and must only be accessed by a qualified professional after isolating mains power.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
- Loweditorial warning
On Duralis and ACI Visualis water heaters, a slow green flash on the outer light every 5 seconds during peak hours is normal operation showing that the ACI anti-corrosion system is running on battery backup.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Symptoms and causes
Probable causes · in order
- Brand-new water heater whose ACI accumulator battery is not yet chargedCommon
On a newly installed unit, the ACI backup battery starts uncharged and requires two 8-hour off-peak heating cycles (16 hours of electrical supply in total) before Err 01 clears automatically.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
- Power switched off during holidays or an extended absence from homeCommon
When electrical power to the water heater is turned off during an absence, the ACI anti-corrosion backup battery fully discharges after 24 hours and triggers Err 01 once power returns.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
- Insufficient daily power supply from the off-peak contactor circuitPossible
The water heater is not receiving at least 8 hours of electrical power per day from the distribution board, or the day/night off-peak contactor is failing to switch power on.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
- Exhausted accumulator integrated into the electronic thermostat assemblyLess common
On a water heater that is not new and still displays the fault after 16 to 24 hours of confirmed continuous power, the battery built into the electronic thermostat assembly can no longer hold a charge.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 01 ? · Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Virtual technician
Does Err 01 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 01 ?
Thermor · 2026 · official_doc
- 3
Le voyant de mon chauffe-eau électrique Duralis clignote, quelle est sa signification ?
Thermor · 2026 · official_doc
Was this entry useful?