- Boilers
- Gas boiler
C4
Junkers C4 on a gas boiler: pressure switch contact does not open
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Junkers C4 means, on the ZWE with Heatronic, that the pressure switch contact does not open. Check for a burnt gas smell, look at the flue outlet from outside, note any changes to the flue and call the technical service.
Safety first
Stop and check this first
- If you smell gas: close the gas valve, open the windows, do not operate any electrical switch and put out any flame, as Bosch asks. Then leave the home and call your gas distributor's emergency number from outside.
- If you smell burnt gases: switch the appliance off, open doors and windows and call an installer, as Bosch says. Carbon monoxide has no smell: with a headache, dizziness or nausea, get out of the home and seek help. Fit a CO alarm near the boiler.
- Do not extend or change the flue pipe yourself: Bosch requires Junkers flue accessories and that installation and modifications are done by an authorised installer.
Safe checks you can perform
- Gas smell, burnt gas smell or symptoms
- Model and recent changes to the flue
- Flue outlet looked at from outside
- Fault reset key, once only
- Pressure switch, fan and flue diagnosis
Leave these tasks to a qualified professional
- Do not attempt this professional task: Fan, control and pressure switch
- Do not attempt this professional task: Combustion products measurement
- Do not attempt this professional task: Safety devices check
- Do not attempt this professional task: Flue route and length check
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.
Fan, control and pressure switch
What they doThe technician checks the operation of the fan, the control that drives it and the differential pressure switch contact.
They must show youthe contact and fan test.
In writingthe faulty part, the genuine part number and why.
Combustion products measurement
What they doAfter the repair, the technician measures CO2 and flue gas temperature at the flue test point and the intake air at its test point, at constant output, as the manual describes.
They must show youthe analyser printout.
In writingthe values measured against the manufacturer's specification and a test run without the code.
Safety devices check
What they doThe technician carries out the operating check of all safety, regulation and control devices that the ZWE maintenance calls for.
They must show youthe boiler starting without codes.
In writingthe checks made and their result.
Flue route and length check
What they doThe technician checks the concentric intake and flue route, confirms the accessories are Junkers and that the flue pipe length meets the technical manual for that flue.
They must show youthe flue route and its accessories.
In writingthe length checked against the manufacturer's specification and what was corrected.
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.
Gas smell, burnt gas smell or symptoms
- Who:
- you.
- How:
- first check whether there is a smell of gas or burnt gases near the boiler, or whether anyone has a headache, dizziness or nausea.
- Result that matters:
- ruling out gas or fumes indoors before going on.
- If you smell gas:
- leave the home without touching switches and call from outside, as the warning says.
- If you smell burnt gases:
- switch the appliance off, open doors and windows and call an installer, as Bosch asks.
- Evidence to keep:
- a note of the time and what you noticed.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Model and recent changes to the flue
- Who:
- you.
- How:
- confirm on the rating plate that it is a ZWE 24-3 or 28-3 H A with Heatronic and recall whether the boiler was installed, the flue extended or work done on the outside wall recently.
- Result that matters:
- whether C4 appeared from installation or after a flue change.
- If it coincides with a flue change:
- tell the technician; it fits the flue pipe length cause.
- If nothing has changed:
- the fan, the control or the air path weigh more.
- Evidence to keep:
- photo of the plate and dates of any work or changes.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Flue outlet looked at from outside
- Who:
- you.
- How:
- find the concentric flue and air terminal on the wall or roof and look at it from below or from a window. Do not touch it or climb on anything.
- Result that matters:
- whether anything covers, bends or has moved it.
- If it is blocked or damaged:
- do not fix it yourself; take photos for the technician.
- If it is clear and intact:
- the inspection moves inside the boiler.
- Evidence to keep:
- photos of the terminal and its surroundings.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Fault reset key, once only
- Who:
- you.
- How:
- if the fault reset key flashes with C4, press it until the display shows "--", as Bosch describes the unlocking. Do it once.
- Result that matters:
- whether the flow temperature reappears and the boiler runs.
- If C4 repeats or the key does not flash:
- leave the boiler as it is and call the technical service; the manual gives the user no other action for C4.
- If it runs:
- note when it happened in case it returns.
- Evidence to keep:
- photo of the display with C4.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Pressure switch, fan and flue diagnosis
- Who:
- Junkers technical service or an authorised company.
- How:
- disconnects the mains and checks the air path, the fan and the control, and the flue pipe length against what the technical manual for that flue allows.
- Result that matters:
- why the pressure switch contact does not open.
- If the flue exceeds what is allowed or is not Junkers:
- the flue installation is corrected.
- If the flue is correct:
- the fan, the control and the pressure switch are checked.
- Evidence to keep:
- job sheet with the length checked, the parts inspected and the combustion reading.
Installation and any modification may only be carried out by an authorised installer, and Bosch reserves all work inside the appliance for the technical service. The user does not open the boiler or touch the flue.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
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
- Gas boiler
Known code variants
- aire acondicionado Daikin C4
- C04
- C 4
- climatisation Daikin C4
- climatizzatore Daikin C4
- code erreur C4 Daikin
- codice C4 Daikin
- código C4 Daikin
- Daikin AC C4
- Daikin C4
- Daikin error C4
- Daikin Fehler C4
- Daikin Fehlercode C4
- error C4 Daikin
- Error C4 Junkers
- errore C4 Daikin
- Junkers C4
- Klimaanlage Daikin C4
- panne C4 Daikin
- ZWE C4
What it means
In the ZWE 24-3 and 28-3 H A fault code table, C4 means the pressure switch contact does not open; this device monitors the air flow through the sealed chamber. Bosch says to check the air path, the fan and the control, and the flue pipe length, which must be built with Junkers accessories. Unlike C1, here the contact does not change state. The code does not tell apart a fault in the flue, the fan, the control or the pressure switch itself.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas: close the gas valve, open the windows, do not operate any electrical switch and put out any flame, as Bosch asks. Then leave the home and call your gas distributor's emergency number from outside.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Higheditorial warning
If you smell burnt gases: switch the appliance off, open doors and windows and call an installer, as Bosch says. Carbon monoxide has no smell: with a headache, dizziness or nausea, get out of the home and seek help. Fit a CO alarm near the boiler.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Mediumsource verified
Do not extend or change the flue pipe yourself: Bosch requires Junkers flue accessories and that installation and modifications are done by an authorised installer.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Symptoms and causes
Probable causes · in order
- Air pathPossible
Bosch lists it first in the C4 checks. If the intake or flue passage does not behave as the control expects, the pressure switch does not change state.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Fan or controlPossible
Bosch says to check the fan and the control. Unlike C1, here the pressure switch contact never opens, so the assembly that drives it is checked.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Flue pipe lengthPossible
Bosch mentions the flue pipe length. It weighs more if the flue was installed or changed recently; only Junkers flue accessories are allowed.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Virtual technician
Does C4 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 ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Junkers (Bosch Termotecnia) · 2000 · manual
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