- Boilers
- Gas boiler
2951
Junkers Bosch 2951 on a gas boiler: too many flame losses
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Junkers Bosch 2951 means the boiler has exceeded the maximum number of flame losses and has locked out. Check that the gas valve is open and press reset once. If it comes back, do not keep trying and call the service technician.
Safety first
Stop and check this first
- If you smell flue gas or suspect a carbon monoxide leak: Bosch says switch the boiler off, open doors and windows, warn the neighbours, leave the building and call the official service. CO has no colour, smell or taste: if you feel headache, dizziness or nausea, get into fresh air and call for help. Bosch recommends CO alarms.
- If you smell gas: Bosch says do not smoke or use lighters or matches, do not operate switches or pull plugs, do not use the phone or doorbell, close the gas supply at the main valve or meter, open doors and windows, warn the neighbours, leave the building and keep others out. From outside, call the fire brigade, the police and the gas company.
Safe checks you can perform
- Gas or flue gas smell near the boiler
- Boiler gas valve open
- One reset and watch
- Flue terminal and condensate pipe in view
- History, gas and ionisation by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flue, condensate and combustion
- Do not attempt this professional task: Fault history reading
- Do not attempt this professional task: Gas pressure at full load
- Do not attempt this professional task: Ionisation and fan
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.
Flue, condensate and combustion
What they doChecks how the flue duct is fitted and how the condensate drain is routed, and finishes with a combustion analysis.
They must show youthe final combustion analysis.
In writingthe CO2 and CO measured against the manufacturer's table.
Fault history reading
What they doThe technician checks the current fault (1-A2) and the history to see which lockout faults came before 2951.
They must show youthe list of earlier faults.
In writingthe codes read and which one was taken as the cause.
Gas pressure at full load
What they doMeasures the gas inlet pressure with the boiler at full load and checks the appliance gas settings.
They must show youthe pressure measured against the manufacturer's specification.
In writingthe value and whether the gas installation needs correcting.
Ionisation and fan
What they doChecks the ionisation cable connections, the ionisation current and the fan's operation and power draw.
They must show youthe measured ionisation current.
In writingthe value and the parts checked or replaced.
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 or flue gas smell near the boiler
- Who:
- you.
- How:
- walk up without pressing switches or turning lights on and notice whether there is a smell of gas or combustion fumes.
- Result that matters:
- whether there is a smell or not.
- If there is a smell:
- do not touch the boiler and follow the gas warning on this page.
- If there is no smell:
- go on to check the gas valve.
- Evidence to keep:
- the time and what you noticed.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Boiler gas valve open
- Who:
- you.
- How:
- look at the boiler gas valve knob; the user manual says it is open with the knob in line with the pipe and closed at a right angle.
- Result that matters:
- whether it was open or closed.
- If it was closed:
- open it by pressing the knob and turning it to the left, as the manual shows, then go to the reset.
- If it was open:
- the gas was not shut off at that point; go to the next step.
- Evidence to keep:
- a photo of the valve and of the display showing 2951.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
One reset and watch
- Who:
- you.
- How:
- press the up and down arrows together until the fault symbols disappear, or switch the boiler off and on, as the user manual says.
- Result that matters:
- whether the boiler starts and whether 2951 returns.
- If it returns:
- do not repeat resets; Bosch says 5 lockouts reset within 15 minutes lead to the 2980 super lockout.
- If it does not return:
- note when it appeared in case it happens again.
- Evidence to keep:
- the reset time and whether the code returned.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266) · Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Flue terminal and condensate pipe in view
- Who:
- you.
- How:
- from the ground or outside, without taking anything apart, check whether the flue terminal is blocked or damaged and whether the condensate pipe from the boiler is loose or kinked.
- Result that matters:
- whether you see anything damaged, loose or blocked.
- If something looks wrong:
- do not handle it and tell the technician; Bosch names badly fitted ducts and drains.
- If everything looks fine:
- the internal checks are for the technician.
- Evidence to keep:
- photos of the terminal and the condensate pipe.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266) · Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
History, gas and ionisation by the technician
- Who:
- authorised technician.
- How:
- reads the fault history, measures the gas pressure at full load, reads the ionisation current in menu 1-C1 and checks the fan and wiring.
- Result that matters:
- which lockout fault caused the flame losses.
- If there is a gas pressure drop:
- the problem is in the gas installation.
- If the gas is fine:
- ionisation, fan or ducts become the suspects.
- Evidence to keep:
- a report with the history, the measured pressure and what was corrected.
Bosch says never remove the boiler casing and leave work to an authorised company. Inside there are gas parts and mains voltage.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
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
- Bosch 2951
- Bosch Condens 2951
- Condens 2300i W 2951
- Condens 4300iW 2951
- Error 2951 Junkers
- Junkers 2951
What it means
On the Bosch Condens 4300iW and 2300i W, 2951 is a locking fault (category V): the maximum number of flame losses has been exceeded. Bosch treats it as the result of other lockout faults, so the technician must read the fault history. As possible causes it lists incomplete purging of the gas pipe, undersized gas pipes or gas pressure drop, badly routed condensate drains, incorrectly fitted flue ducts and incorrect wiring. The code does not point to a single part.
Warnings and stop conditions
- Higheditorial warning
If you smell flue gas or suspect a carbon monoxide leak: Bosch says switch the boiler off, open doors and windows, warn the neighbours, leave the building and call the official service. CO has no colour, smell or taste: if you feel headache, dizziness or nausea, get into fresh air and call for help. Bosch recommends CO alarms.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
- Criticalsource verified
If you smell gas: Bosch says do not smoke or use lighters or matches, do not operate switches or pull plugs, do not use the phone or doorbell, close the gas supply at the main valve or meter, open doors and windows, warn the neighbours, leave the building and keep others out. From outside, call the fire brigade, the police and the gas company.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Symptoms and causes
Probable causes · in order
- Not enough gas in the supplyCommon
Bosch names undersized gas pipes or a gas pressure drop as causes of the flame being lost again and again.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Air in the gas pipePossible
Bosch says the gas pipe may not have been fully purged. This is more likely with a newly installed boiler or after work on the gas installation.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Badly fitted condensate drain or fluePossible
Bosch mentions badly routed condensate drains and incorrectly fitted flue ducts as a source of repeated flame losses.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Ionisation, fan or wiringPossible
For the 2300i W, Bosch says to check the ionisation cable and connectors, the ionisation current and the fan; for the 4300iW it also names incorrect wiring.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Virtual technician
Does 2951 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
Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700)
Bosch Termotecnia (Robert Bosch España) · 2025 · manual
- 2
Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Bosch Termotecnia (Robert Bosch España) · 2021 · manual
- 3
Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Bosch Termotecnia (Robert Bosch España) · 2021 · manual
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