- Boilers
- Gas boiler
2971
Junkers Bosch 2971 on a gas boiler: water pressure too low
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Junkers Bosch 2971 means the system's operating pressure is too low. Press OK to see it: cold, it should be between 1 and 2 bar. If it is low, refill with the system cold; if it drops again, call the technician.
Safety first
Stop and check this first
- 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.
- Do not refill a hot boiler: Bosch warns that cold water in a hot boiler can cause thermal stress cracks. Fill only when the system is cold, with water at 40 °C at most, and never above 3 bar at maximum temperature.
Safe checks you can perform
- Gas or flue gas smell near the boiler
- Operating pressure on the display
- Cold refill with the blue lever
- Visible leaks
- Leaks, vessel and sensor by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: System leak test
- Do not attempt this professional task: Expansion vessel check
- Do not attempt this professional task: Pressure sensor
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.
System leak test
What they doThe technician finds where water is lost in the heating system and in the boiler.
They must show youwhere the leak was.
In writingthe leak found and the repair.
Expansion vessel check
What they doChecks the boiler expansion vessel for leaks.
They must show youthe vessel check.
In writingits condition and whether any work was done on it.
Pressure sensor
What they doChecks the harness continuity and the voltage at the pressure sensor, and compares its reading with the real pressure.
They must show youthe measured voltage.
In writingthe result and whether the sensor was 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 read the pressure.
- Evidence to keep:
- the time and what you noticed.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Operating pressure on the display
- Who:
- you.
- How:
- with the system cold, press the OK key; the user manual says the display then shows the operating pressure.
- Result that matters:
- whether it is below 1 bar, between 1 and 2 bar, or whether LoPr appears.
- If it is below 1 bar or LoPr shows:
- water is missing; go to the refill.
- If it is between 1 and 2 bar and 2971 remains:
- the sensor reading becomes more likely; call the technician.
- Evidence to keep:
- a photo of the display with the pressure and with 2971.
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)
Cold refill with the blue lever
- Who:
- you, or the technician; Bosch recommends leaving filling to a qualified technician.
- How:
- with the system cold, push the blue filling lever down and release it when the pressure reaches the target, between 1 and 2 bar.
- Result that matters:
- whether 2971 clears and the pressure holds for several days.
- If it drops again:
- Bosch says to check the expansion vessel and the system for leaks; call the technician.
- If you overshoot:
- stop; when hot it must never exceed 3 bar.
- Evidence to keep:
- the pressure before and after, and the date.
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)
Visible leaks
- Who:
- you.
- How:
- without opening the boiler, look underneath it and at radiators, valves and exposed pipes for drips, stains or limescale marks.
- Result that matters:
- whether you see water or signs of a leak.
- If you see a leak:
- note it and tell the technician.
- If you see nothing:
- the loss may be hidden or in the expansion vessel; the technician checks it.
- Evidence to keep:
- photos and the pressure noted daily for a week.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Leaks, vessel and sensor by the technician
- Who:
- authorised technician.
- How:
- checks the system and the expansion vessel for leaks, and checks the pressure sensor harness and voltage.
- Result that matters:
- whether there is a real water loss or a false reading.
- If there is a loss:
- it must be repaired before refilling again.
- If there is no loss and the reading does not match:
- the sensor becomes the suspect.
- Evidence to keep:
- a report with the cause and the final cold pressure.
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 2971
- Bosch Condens 2971
- Condens 2300i W 2971
- Condens 4300iW 2971
- Error 2971 Junkers
- Junkers 2971
What it means
On the Bosch Condens 4300iW and 2300i W, 2971 means insufficient operating pressure. Bosch says to purge the system and fill it to the set pressure and, if it persists, check the pressure sensor: its harness continuity and the voltage at the sensor. The installation manual gives 1 bar as the cold minimum and 1 to 2 bar as the optimum. The user manual also describes the LoPr message: below 0.3 bar the heating is blocked. If the pressure does not hold, Bosch says to check the expansion vessel and the system for leaks.
Warnings and stop conditions
- 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)
- Highsource verified
Do not refill a hot boiler: Bosch warns that cold water in a hot boiler can cause thermal stress cracks. Fill only when the system is cold, with water at 40 °C at most, and never above 3 bar at maximum temperature.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Symptoms and causes
Probable causes · in order
- Water loss in the systemCommon
If the pressure does not hold, Bosch says to check the heating system for leaks. A small leak at a radiator or valve lowers the pressure bit by bit.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Air bled from radiatorsPossible
Bosch says to purge air and fill to the set pressure. Bleeding radiators lets air out and lowers the pressure, so check it again after bleeding.
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)
- Expansion vesselPossible
When the pressure does not hold, Bosch includes the expansion vessel in the leak check. The technician checks it, never the user.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Pressure sensor or its wiringLess common
If the real pressure is correct, Bosch says to check the pressure sensor harness continuity and the voltage at the sensor, and replace it if needed.
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 2971 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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