- Boilers
- Gas boiler
C1
Junkers C1 on a gas boiler: pressure switch opens or fan too slow
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Junkers C1 means, on the ZWE with Heatronic, that the pressure switch opens while the boiler is running or that the fan turns too slowly. Check for a burnt gas smell, look at the flue terminal from outside and call the technical service.
Safety first
Stop and check this first
- If you smell gas: Bosch says to close the gas valve, open the windows, not to operate any electrical switch, to put out any flames and to call the gas company. Do it without using anything that sparks, leave the home and call your distributor's emergency number from outside.
- If you smell burnt gases: Bosch says to switch the appliance off, open doors and windows and call an installer. Carbon monoxide has no smell; if anyone has a headache, dizziness or nausea, get outside and seek help. A CO alarm near the boiler gives early warning.
- Do not touch the flue or the pressure switch tube: Bosch forbids modifying the parts that carry burnt gases and reserves all work inside the appliance for the technical service.
Safe checks you can perform
- Gas smell, burnt gas smell or symptoms
- Exact model on the rating plate
- Flue terminal seen from outside
- One attempt with the fault reset key
- Fan, pressure switch and air path checked by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Differential pressure switch test
- Do not attempt this professional task: Fan inspection
- Do not attempt this professional task: Air and flue duct and combustion analysis
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.
Differential pressure switch test
What they doThe technician checks the differential pressure switch and its pressure tapping with the boiler running, to see whether the contact opens during operation.
They must show youthe pressure switch, its tubes and the contact test.
In writingwhether it was fine or replaced, with the part number and why.
Fan inspection
What they doThe technician checks that the fan turns normally and receives the correct supply from the circuit board.
They must show youthe fan running or the fault found.
In writingthe diagnosis and, if replaced, the genuine part number.
Air and flue duct and combustion analysis
What they doThe technician checks the intake and flue route, which must use Junkers accessories, and measures CO2, flue gas temperature and intake air at the boiler test points.
They must show youthe analyser printout.
In writingthe values measured against the manufacturer's specification and what was corrected in the duct.
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:
- before touching anything, notice whether there is a smell of gas or burnt gases near the boiler, or whether anyone at home has a headache, dizziness or nausea.
- Result that matters:
- whether there are signs of gas or fumes indoors.
- If you smell gas:
- follow the gas warning, leave the home and call from outside.
- If you smell burnt gases:
- Bosch says to switch the appliance off, open doors and windows and call an installer.
- 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
Exact model on the rating plate
- Who:
- you.
- How:
- check on the rating plate or the front panel whether your boiler is a ZWE 24-3 or ZWE 28-3 H A with Heatronic.
- Result that matters:
- confirming that the C1 meaning in this table applies to your appliance.
- If it is a ZWE with Heatronic:
- C1 is the pressure switch opening during operation or a slow fan.
- If it is another Junkers model:
- the code may mean something else; tell the technician.
- Evidence to keep:
- photo of the rating plate.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Flue terminal seen from outside
- Who:
- you.
- How:
- look at the flue outlet and air inlet terminal from the street, the ground or a window, without touching it, taking anything apart or climbing.
- Result that matters:
- whether it is blocked, crushed, broken or covered by something (a nest, sheeting, ice, building work).
- If something looks wrong:
- do not clear it yourself; photograph it and tell the technician, it fits the air path cause.
- If it looks clear:
- the cause may be inside, in the fan or the pressure switch.
- Evidence to keep:
- photos of the terminal from where you can see it.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
One attempt with the fault reset key
- Who:
- you.
- How:
- Bosch describes that, in a lockout, the fault reset key flashes and is pressed until the display shows "--". If it flashes with C1, make one attempt that way.
- Result that matters:
- whether the boiler goes back to showing the flow temperature and runs without C1.
- If C1 returns or the key does not flash:
- do not keep trying; the manual gives the user no other reset for C1. Call the technical service.
- If it starts:
- note the date and time and still tell the technician if it repeats.
- Evidence to keep:
- photo of the display with C1 and a note of when it appeared.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Fan, pressure switch and air path checked by the technician
- Who:
- Junkers technical service or an authorised company.
- How:
- with the boiler disconnected from the mains, checks the differential pressure switch and its pressure tapping, the fan and the air path, as the fault table says.
- Result that matters:
- which of the three makes the pressure switch open during operation.
- If the fan runs slowly:
- its supply is checked or it is replaced with a genuine part.
- If the fan is fine:
- the pressure switch and the intake and flue duct are checked.
- Evidence to keep:
- job sheet with what was checked, the part replaced and the final combustion reading.
Bosch reserves all work inside the appliance for the technical service and forbids modifying the parts that carry burnt gases. Fan, pressure switch and flue are technician-only work.
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
- AEG C4 steam pyro
- AEG C4 vapor pirólisis
- AEG horno C1
- AEG horno error C2 sonda
- AEG oven C1
- AEG oven C3 door pyro clean
- AEG oven error C2 food sensor
- AEG pirólisis C1
- AEG pyrolytic C1
- C0
- C01
- C 1
- C2
- C3
- C4
- error C0 C1 C2 C3 placa Samsung
- Error C1 Junkers
- horno AEG C3 puerta pirólisis
- Junkers C1
- placa inducción Samsung C1
- Samsung C1
- Samsung cooktop C0 C1 C2 C3
- Samsung cooktop error C1
- Samsung hob sensor error C1
- sensor temperatura encimera Samsung
- ZWE C1
What it means
In the ZWE 24-3 and 28-3 H A fault code table, C1 means the pressure switch opens during operation or the fan speed is too low. The differential pressure switch monitors the air the fan moves through the sealed chamber and the concentric flue. Bosch says to check the pressure switch, the fan and the air path. The code does not say which of the three is at fault: it may be the part that measures, the part that moves the air or the duct itself.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas: Bosch says to close the gas valve, open the windows, not to operate any electrical switch, to put out any flames and to call the gas company. Do it without using anything that sparks, leave the home and call your 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: Bosch says to switch the appliance off, open doors and windows and call an installer. Carbon monoxide has no smell; if anyone has a headache, dizziness or nausea, get outside and seek help. A CO alarm near the boiler gives early warning.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Mediumsource verified
Do not touch the flue or the pressure switch tube: Bosch forbids modifying the parts that carry burnt gases and reserves all work inside the appliance for the technical service.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Symptoms and causes
Probable causes · in order
- Fan turning too slowlyPossible
Bosch names it in the C1 description itself: the fan speed is too low. The boiler starts, but the air it moves is not enough.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Differential pressure switch opening during operationPossible
The pressure switch confirms the air flow. If its contact opens while the boiler is already running, the control shuts down. Bosch says to check it first.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
- Combustion air pathPossible
Bosch lists the air path among the C1 checks. A restricted intake or flue passage affects the fan and the pressure switch at the same time.
Evidence: Junkers ZWE 24-3 y 28-3 H A con Bosch Heatronic, instrucciones de instalación
Virtual technician
Does C1 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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