- Boilers
- Tankless gas water heater
A0
Junkers A0 on gas water heater: inlet and outlet sensors
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
On Junkers Hydronext 5700S and 5600S thermostatic gas water heaters, code A0 means both the water inlet and water outlet temperature sensors are damaged or disconnected. Check live readings 1d and 2d in menu P4 without opening the casing, then call a qualified technician.
Safety first
Stop and check this first
- The water outlet temperature sensor is one of the appliance's safety devices (Section 3.6); never open the casing or attempt to bypass or remove either temperature probe yourself.
- Removing water sensors carries a risk of water leaks unless the water and gas shut-off valves are closed first and disturbed O-rings are replaced with new ones by an authorised technician.
Safe checks you can perform
- Check live sensor readings 1d and 2d in the P4 -> Od information menu
- Try a single reset by holding the return symbol for 5 seconds
- Switch the heater off with ON/OFF and book a qualified technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Inspect and test the inlet and outlet water temperature sensors and their wiring
- Do not attempt this professional task: Replace defective sensors with genuine parts and renew hydraulic seals
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 and test the inlet and outlet water temperature sensors and their wiring
What they doThe qualified technician closes the water and gas shut-off devices, removes the 2 front cover screws, and tests the water inlet temperature sensor (item 14 in Figure 2 / item 10 in Figure 26), the water outlet temperature sensor (item 8 / item 9) and their plugs at the control box.
They must show youthe condition of both sensor plugs at the control box and the live 1d and 2d temperature readings in menu Od.
In writingthe resistance measurements for the inlet and outlet sensors and whether the fault lay in the wiring or the probes.
Replace defective sensors with genuine parts and renew hydraulic seals
What they doIf either temperature probe has failed, the Technical Assistance Service fits genuine Junkers replacement sensors, installs new gaskets and O-rings lubricated with Unisilikon L 641 on hydraulic joints (Chapter 9), and checks for water leaks and proper thermostatic control.
They must show youthe replaced sensor components, leak-free water inlet and outlet joints, and stable outlet temperature regulation.
In writingthe genuine part numbers fitted, confirmation of new hydraulic seals, and the completed safety and function test.
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 live sensor readings 1d and 2d in the P4 -> Od information menu
- Who:
- you, using only the touch symbols on the outside display.
- How:
- press the menu symbol until P4 appears, confirm to enter Od (Operating data), and scroll to inspect parameter 1d (current water inlet pipe sensor temperature) and 2d (current water outlet pipe sensor temperature), as well as 1F under FH (Fault history).
- Result that matters:
- checking from outside what values the two water temperature sensors report and confirming A0 as the logged fault.
- If the screen shows E2 or A7 instead of A0:
- only one water temperature sensor is affected (inlet for E2, outlet for A7).
- If A0 is confirmed along with abnormal 1d and 2d readings:
- write down both numbers for the visiting technician.
- Evidence to keep:
- photos of the display showing code A0 and the parameter values under 1d and 2d.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
Try a single reset by holding the return symbol for 5 seconds
- Who:
- you, once only from the external control panel.
- How:
- with hot water taps closed, press and hold the return symbol for 5 seconds as described in the Resetting the appliance section (4.9 on 5700S; 4.10 on 5600S) to see whether the safety lockout clears.
- Result that matters:
- verifying whether A0 was a one-off start-up glitch or a persistent sensor fault.
- If A0 clears and the heater delivers hot water normally:
- keep an eye on the display over the next few draws.
- If A0 returns immediately:
- do not repeat the reset; proceed to switch the appliance off for professional service.
- Evidence to keep:
- note of the heater's behaviour after holding the return symbol for 5 seconds.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
Switch the heater off with ON/OFF and book a qualified technician
- Who:
- you, without loosening the front casing screws.
- How:
- press the ON/OFF symbol on the display to switch the water heater off, close the gas shut-off valve if the unit will stay out of use, and contact a qualified technician or Official Junkers Service.
- Result that matters:
- leaving the appliance safely shut down until a professional inspects the inlet and outlet sensors.
- If the unit is off awaiting service:
- note the model (WTD 12, 15 or 17) from the bottom rating plate.
- If freezing temperatures are likely while the heater is disconnected without an anti-freeze kit:
- follow the draining procedure in Section 4.7 / 4.8.
- Evidence to keep:
- model number, 10-digit reference, FD code and your 1d/2d notes.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
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
- Junkers
- Product type
- Tankless gas water heater
Known code variants
- A 0
- A0
- A1
- A2
- A3
- A4
- A5
- A6
- A7
- A8
- A9
- AO
- Bosch A0
- C0
- C1
- C2
- C3
- C4
- C5
- C6
- C7
- calentador Junkers A0
- CO
- error A0 calentador Junkers
- Hydronext 5600S A0
- Hydronext 5700S A0
- Junkers A0
- Junkers gas water heater A0
- Junkers water heater A0 fault
What it means
In the fault tables of the official manuals for the Junkers Hydronext 5700S (WTD 12/15/17-4AME, document 6720884104) and Hydronext 5600S (WTD 12/15/17-3AME, document 6721841217) room-sealed thermostatic gas water heaters, display code A0 indicates that the water inlet and outlet temperature sensors are damaged (Sensor de temperatura de entrada y salida dañados) and triggers a safety lockout. The same fault table lists E2 when only the inlet sensor (item 14) fails and A7 when only the outlet sensor (item 8) fails. Other models such as the 2400S may differ. On its own, A0 never proves both probes have failed, as a shared harness or control box issue can affect both signals.
Warnings and stop conditions
- Higheditorial warning
The water outlet temperature sensor is one of the appliance's safety devices (Section 3.6); never open the casing or attempt to bypass or remove either temperature probe yourself.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
- Mediumeditorial warning
Removing water sensors carries a risk of water leaks unless the water and gas shut-off valves are closed first and disturbed O-rings are replaced with new ones by an authorised technician.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
Symptoms and causes
Probable causes · in order
- Loose plug or damaged wiring harness between both water sensors and the control boxCommon
In the wiring diagram (Figure 26), the outlet water temperature sensor (item 9) and inlet water sensor (item 10) connect to the control box, so a disturbed harness plug affects both readings at once.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
- Simultaneous failure of the cold-water inlet and hot-water outlet temperature probesPossible
The troubleshooting table in both Hydronext manuals defines A0 as damaged inlet (item 14 in Figure 2) and outlet (item 8) water temperature sensors.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
- Fault in the sensor evaluation circuit of the electronic control boxLess common
If both temperature probes and their leads test within specification, a fault inside the control box (item 13) can prevent the heater from reading both sensors.
Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)
Virtual technician
Does A0 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
Junkers (Bosch Termotecnia) · 2022 · manual
- 2
Junkers (Bosch Termotecnia) · 2021 · manual
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