- Boilers
- Tankless gas water heater
E4
Junkers E4 gas water heater: flue gas leak, casing overheat
Risk level
High
Professional inspection is recommended.
Urgency
Immediate
Quick answer
E4 on a Junkers Hydronext 5700S or 5600S means the casing air sensor has detected overheating caused by a possible leak of combustion products. Switch the heater off, ventilate the room and call a gas technician.
Safety first
Stop and check this first
- If you notice any smell of gas, avoid sparks and flames: do not press switches, unplug anything, phone or ring the doorbell indoors. Close the main gas valve, open doors and windows, warn neighbours as you leave, keep others out and call the fire service and gas company from the street.
- E4 points to combustion gases inside the appliance, with a risk of carbon monoxide (CO), a colourless and odourless gas. Do not reset or use the heater: switch it off, ventilate and, if there are symptoms or a CO alarm, leave the home and call the service technician.
Safe checks you can perform
- Switch the heater off at the panel
- Ventilate and watch for CO symptoms
- Read the fault history without resetting
- Combustion leak diagnosis
Leave these tasks to a qualified professional
- Do not attempt this professional task: Locate the combustion product leak
- Do not attempt this professional task: Check the casing air sensor
- Do not attempt this professional task: Combustion test before handover
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.
Locate the combustion product leak
What they doA qualified gas technician inspects the combustion chamber, its seals and its joint with the flue to find where gases are escaping.
They must show youthe damaged seal or section that caused the heat.
In writingthe part repaired or replaced and the tightness test carried out.
Check the casing air sensor
What they doA qualified gas technician tests the casing air temperature sensor and its connection to the control box.
They must show youthe sensor reading against the actual temperature.
In writingthe sensor condition and whether anything was replaced.
Combustion test before handover
What they doA qualified gas technician measures combustion and confirms correct flue operation with the front panel fitted after the repair.
They must show youthe analyser reading and the flue check.
In writingthe measured values and the conclusion that it is safe to use.
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.
Switch the heater off at the panel
- Who:
- you, at the on/off switch.
- How:
- press the on/off switch to disconnect the appliance, as the table instructs for E4, and do not switch it back on to test it.
- Result that matters:
- the burner staying off until the technician arrives.
- If confirmed:
- also close the heater gas valve if you can reach it.
- If not confirmed:
- if it will not switch off, close the gas valve and call urgently.
- Evidence to keep:
- time E4 appeared and time you switched the heater off.
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)
Ventilate and watch for CO symptoms
- Who:
- you and everyone at home.
- How:
- open the doors and windows of the heater room and watch for headache, dizziness or nausea; if you have a CO detector, check whether it has alarmed.
- Result that matters:
- whether there are signs that flue gases reached the living space.
- If confirmed:
- get everyone out, warn neighbours and call the service technician from outside.
- If not confirmed:
- keep the heater off until it is inspected.
- Evidence to keep:
- whether the CO alarm sounded and whether anyone had symptoms.
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)
Read the fault history without resetting
- Who:
- you, using the user menu.
- How:
- open the FH menu on the panel and note the stored codes (1F to 10F) to see whether E4, A1 or A4 have appeared before; do not reset the appliance.
- Result that matters:
- knowing whether E4 is new or recurring.
- If confirmed:
- show the stored E4 history to the technician.
- If not confirmed:
- tell them it is the first time you have seen it.
- Evidence to keep:
- photo of the FH fault list.
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)
Combustion leak diagnosis
- Who:
- a qualified gas technician attending the E4 warning.
- How:
- with the heater cold, inspects the combustion chamber, seals, flue and the casing air sensor with its connection, and checks tightness before putting the heater back into service.
- Result that matters:
- whether combustion products are leaking and where.
- If confirmed:
- restores tightness and verifies combustion with the front panel fitted.
- If not confirmed:
- tests the sensor and its connection rather than returning the heater unexplained.
- Evidence to keep:
- report with the leak point, the repair and the tightness test.
With E4 there may be combustion gases inside the casing; only a gas technician opens the appliance, and only once it is cold and isolated.
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
- Bosch E4
- calentador Junkers E4
- Daitsu E4
- E04
- E 4
- E401
- E402
- E403
- E421
- E422
- E423
- E431
- E432
- E433
- Electrolux cooker E4
- Electrolux hob E4
- Electrolux hob E401 overheating
- Electrolux induction hob error E4
- Electrolux placa E4
- error E4 lavadora Haier
- error E4 lavavajillas Fagor
- error E4 placa inducción Electrolux
- Fagor 3LVF-623.1 E4
- Fagor dishwasher E4
- Fagor dishwasher error E4
- Fagor dishwasher overflow E4
- Fagor lavavajillas E4
- Haier E4 error washer
- Haier E4 lavadora
- Haier washer E4 not filling
- Haier washing machine E4
- Hydronext 5600S E4
- Hydronext 5700S E4
- Junkers E4
- lavadora Haier E4 no coge agua
- lavavajillas Fagor rebose E4
- Mitsubishi E4
- Mitsubishi Electric E4
- Mr. Slim E4
- PKA E4
- placa Electrolux E401 sobrecalentamiento
- vitro Electrolux E4
What it means
On the Hydronext 5700S and 5600S, E4 reports that the casing air temperature sensor has detected overheating, which Bosch links to combustion products leaking inside the combustion chamber. The table remedy is to switch the appliance off and contact a technician. No user check is listed and resetting to keep using the heater is not offered. This differs from A1, where the heater simply reduces its output.
Warnings and stop conditions
- Criticaleditorial warning
If you notice any smell of gas, avoid sparks and flames: do not press switches, unplug anything, phone or ring the doorbell indoors. Close the main gas valve, open doors and windows, warn neighbours as you leave, keep others out and call the fire service and gas company from the street.
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)
- Criticaleditorial warning
E4 points to combustion gases inside the appliance, with a risk of carbon monoxide (CO), a colourless and odourless gas. Do not reset or use the heater: switch it off, ventilate and, if there are symptoms or a CO alarm, leave the home and call the service 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
- Combustion products leaking in the chamberCommon
Bosch attributes E4 to combustion products escaping inside the combustion chamber, which heats the air within the casing. The joint, duct or part they escape from has to be located.
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)
- Casing air sensor readingLess common
The code comes from the casing air temperature sensor; the same table uses A4 when that sensor is damaged. The technician must confirm the reading is genuine before ruling out a leak.
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 E4 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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