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F0

Junkers F0 on a gas boiler: internal electronics fault

Verified with sources1 sourceReviewed Oct 4, 2026
Technician coming? See the 3 steps they must show you
Wall-mounted combi gas boiler installed in a home
Kai KemmannCC BY-SA 4.0Image source

Risk level

High

Professional inspection is recommended.

Urgency

High

Quick answer

Junkers F0 means an internal boiler fault. If you smell gas or flue gases, follow the warning; if not, reset once as Bosch describes. If F0 returns, leave the boiler off and call the official technical service.

Safety first

Stop and check this first

  • If you get a headache, dizziness or nausea at home while the boiler runs, it may be carbon monoxide: switch the boiler off, ventilate, leave the home and get help. A CO alarm near the boiler warns before symptoms start.
  • Do not chain resets with F0: if it returns after a reset, leave the boiler off. Do not open the casing or touch leads, the board or gas settings.
  • If you smell gas: close the gas shut-off valve, open the windows, do not operate electrical switches, put out any naked flame and call the gas company and an authorised installer from outside the home. If you smell flue gases: switch the boiler off, open doors and windows and tell the installer.

Safe checks you can perform

  • Gas or flue gas smell before anything else
  • One reset with the reset key
  • When F0 appears
  • Contacts, ignition and board check by the technician

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Contacts and ignition leads check
  • Do not attempt this professional task: Diagnosis before replacing the board
  • Do not attempt this professional task: Gas/air ratio 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.

  1. Contacts and ignition leads check

    What they doWith the boiler isolated, the technician checks connectors and ignition leads for loose or corroded contacts or damaged leads.

    They must show youthe connector or lead that was faulty.

    In writingwhat was corrected.

  2. Diagnosis before replacing the board

    What they doThe technician only replaces the circuit board if contacts and leads are fine and F0 persists.

    They must show youthe checks done first.

    In writingthe new board reference and why it was replaced.

  3. Gas/air ratio analysis

    What they doUsing a combustion analyser, the technician measures the flue gases and checks the gas/air ratio against the Bosch specification.

    They must show youthe analyser printout.

    In writingthe measured values against the manufacturer's specification.

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.

  1. Gas or flue gas smell before anything else

    No special tools

    Who:
    you.
    How:
    without touching switches, notice whether there is a smell of gas or flue gases near the boiler.
    Result that matters:
    whether there is a smell.
    If you smell gas or flue gases:
    do not reset; follow the warning on this page.
    If there is no smell:
    you can go on to the reset.
    Evidence to keep:
    a note of the time and what you noticed.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  2. One reset with the reset key

    No special tools

    Who:
    you.
    How:
    if the reset key is flashing, hold it until the display changes; if it is not flashing, switch the boiler off and on again, as Bosch describes. Do it once.
    Result that matters:
    whether the boiler shows the flow temperature again and F0 stays away.
    If it works again:
    note the date and time; if F0 repeats another day, tell the technical service.
    If F0 returns:
    leave the boiler off; Bosch says to contact the official technical service with the fault and the appliance details.
    Evidence to keep:
    a photo of the display with F0 and of the data plate.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  3. When F0 appears

    No special tools

    Who:
    you.
    How:
    note whether F0 comes up when the boiler tries to light, while it is already running, or after a storm or power cut.
    Result that matters:
    when it trips.
    If it trips on lighting:
    tell the technician; ignition leads are on the Bosch list.
    If it trips while running or after a power cut:
    it points more to contacts or the board.
    Evidence to keep:
    a note of the date, time and what the boiler was doing.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  4. Contacts, ignition and board check by the technician

    Tools required

    Who:
    authorised gas technician.
    How:
    opens the boiler, checks contacts and ignition leads, the board and the gas/air ratio with an analyser.
    Result that matters:
    which of the Bosch checks fails.
    If a contact or lead is faulty:
    it is corrected without replacing the board.
    If all is fine and F0 persists:
    the board becomes the suspect.
    Evidence to keep:
    a report of what was checked, the analyser printout and any part replaced.

    Opening the boiler or touching ignition leads, the board or gas settings is work for an authorised gas technician only.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

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

  • CerapurComfort F0
  • Error F0 Junkers
  • F 0
  • FO
  • Gree F0
  • Junkers F 0
  • Philips air purifier F0
  • Philips purificador F0
  • Philips purifier FO clean pre-filter
  • purificador Philips FO limpiar prefiltro

What it means

In the CerapurComfort fault table, F0 is an internal fault. Bosch says to check the electrical contacts and the ignition leads, replace the circuit board if needed and check the gas/air ratio. The Heatronic control watches the safety, regulating and control devices, and F0 appears when something in that chain does not respond as expected. The code does not say whether a contact, an ignition lead, the board or the gas and air setting is at fault.

Warnings and stop conditions

  • Higheditorial warning

    If you get a headache, dizziness or nausea at home while the boiler runs, it may be carbon monoxide: switch the boiler off, ventilate, leave the home and get help. A CO alarm near the boiler warns before symptoms start.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  • Higheditorial warning

    Do not chain resets with F0: if it returns after a reset, leave the boiler off. Do not open the casing or touch leads, the board or gas settings.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  • Criticalsource verified

    If you smell gas: close the gas shut-off valve, open the windows, do not operate electrical switches, put out any naked flame and call the gas company and an authorised installer from outside the home. If you smell flue gases: switch the boiler off, open doors and windows and tell the installer.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

Symptoms and causes

Probable causes · in order

  • Electrical contacts or ignition leadsPossible

    Bosch says to check the electrical contacts and the ignition leads first. A loose connector or damaged lead shows up once the boiler is open, without replacing parts.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  • Circuit boardPossible

    Bosch says to replace the circuit board only if needed. It fits when contacts and leads are fine and F0 keeps appearing.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

  • Gas/air ratio out of adjustmentPossible

    Bosch includes checking the gas/air ratio. It stands apart because it only shows when combustion is measured with an analyser, not by eye.

    Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

Virtual technician

Does F0 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.

Start diagnosis

Sources and technical references

  1. 1

    Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico

    Junkers (Bosch Termotecnia) · 2012 · manual

Sources: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnicoreviewed 2026-10-04 · verified

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