- Boilers
- Gas boiler
A1
Junkers A1 on a gas boiler: pump blocked or running on air
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Junkers A1 means the pump is blocked or running on air, usually because there is too little water in the system. Check the gauge: if it reads below 1 bar, refill to 1 to 1.5 bar with the filling key and press reset once.
Safety first
Stop and check this first
- If you smell gas: Bosch says do not smoke or use lighters, do not operate electrical switches or use the phone or doorbell, close the gas valve at the meter or regulator, open doors and windows, warn the neighbours and leave the building. Call your gas distributor's emergency number from the street.
- Do not leave the boiler running with an empty system: Bosch says it must never be switched on with the appliance or system empty or partly filled. Refill first or wait for the technician.
Safe checks you can perform
- Gas or flue gas smell near the boiler
- Pressure on the boiler gauge
- Refilling with the filling key
- Bleeding radiators that are cold at the top
- Pump and system by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Pump test from the test menu
- Do not attempt this professional task: Search for water loss
- Do not attempt this professional task: Information menu reading
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.
Pump test from the test menu
What they doThe technician runs test t3: the pump runs for up to 45 seconds and, if it is fine, it can be heard.
They must show youtest t3 running and the result.
In writingwhether the pump started and what was done with it.
Search for water loss
What they doChecks the heating system and the boiler for leaks when the pressure drops for no reason, as Bosch asks for permanent drops.
They must show youwhere the leak was, or that there is none.
In writingthe leak point, the repair and the final pressure.
Information menu reading
What they doReads i1 current status and i2 last error to see whether A1 281 repeats or comes with other codes.
They must show youthe display with the last error.
In writingthe codes and cause numbers read.
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 gauge.
- Evidence to keep:
- the time and what you noticed.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Pressure on the boiler gauge
- Who:
- you.
- How:
- look at the needle of the gauge on the front with the heating off, without opening anything.
- Result that matters:
- whether it reads below 1 bar, between 1 and 1.5 bar or above.
- If it reads below 1 bar:
- Bosch says the system needs refilling; go to the next step.
- If it is between 1 and 1.5 bar:
- lack of water becomes less likely; air or the pump become more likely.
- Evidence to keep:
- a photo of the gauge and of the display showing A1 and 281.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Refilling with the filling key
- Who:
- you.
- How:
- remove the lower panel as the manual shows, push the blue lever down and release it when the needle is between 1 and 1.5 bar. Then press reset once.
- Result that matters:
- whether the pressure holds and A1 clears.
- If you go above 1.5 bar:
- Bosch says to drain a little from one radiator until it is back to 1 to 1.5 bar.
- If the pressure drops again within days:
- Bosch says to call the installer about a permanent drop.
- Evidence to keep:
- the pressure before and after, and the refill date.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Bleeding radiators that are cold at the top
- Who:
- you, if the installer has shown you how.
- How:
- with the boiler off, open the top screw of a radiator that is cold at the top a little until water comes out, then check the gauge again.
- Result that matters:
- whether air came out and how far the pressure drops afterwards.
- If air came out:
- air was part of the problem; refill again to 1 to 1.5 bar.
- If you have to bleed often or see leaks:
- Bosch says the technician should inspect the system.
- Evidence to keep:
- which radiators you bled and the final pressure.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Pump and system by the technician
- Who:
- authorised technician.
- How:
- runs pump test t3 from the test menu, reads the information menu and looks for leaks in the system.
- Result that matters:
- whether the pump starts and whether the circuit is losing water.
- If the pump cannot be heard in the test:
- a blocked or faulty pump is confirmed.
- If the pump works and the pressure drops:
- the cause is a leak in the system.
- Evidence to keep:
- a report with the test done, the pressure measured and the leak found.
Bosch reserves removing the casing and any internal work for a competent technician. The pump is inside the boiler, next to mains voltage parts.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
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
- A01
- A 1
- A1 281
- A1 Worcester
- aire acondicionado Daikin A1
- Cerapur A1
- climatisation Daikin A1
- climatizzatore Daikin A1
- code erreur A1 Daikin
- codice A1 Daikin
- código A1 Daikin
- Daikin A1
- Daikin AC A1
- Daikin error A1
- Daikin Fehler A1
- Daikin Fehlercode A1
- E:A1
- EA1
- error A1 Daikin
- Error A1 Junkers
- errore A1 Daikin
- Fujitsu A1
- Fujitsu E A1
- Greenstar A1
- Junkers A1
- Klimaanlage Daikin A1
- panne A1 Daikin
- Worcester A1
- Worcester boiler A1 fault
- Worcester Bosch A1
What it means
On the Cerapur Excellence Compact, A1 with number 281 means pump blocked or running on air, and Bosch gives the possible cause as little or no water in the system. The modulating pump needs a full, air free circuit to move the water. The user manual says to refill if the gauge drops below 1 bar and to bleed the radiators if air gets in. The code does not tell lack of water, air in the circuit or a seized pump apart: the technician settles that.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas: Bosch says do not smoke or use lighters, do not operate electrical switches or use the phone or doorbell, close the gas valve at the meter or regulator, open doors and windows, warn the neighbours and leave the building. Call your gas distributor's emergency number from the street.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
- Highsource verified
Do not leave the boiler running with an empty system: Bosch says it must never be switched on with the appliance or system empty or partly filled. Refill first or wait for the technician.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
Symptoms and causes
Probable causes · in order
- Little or no water in the systemCommon
This is the cause Bosch writes next to code 281. You can tell because the boiler gauge reads below 1 bar or drops to zero.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación · Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
- Air in the heating circuitPossible
The code description mentions a pump running on air. It fits if radiators are cold at the top and hot at the bottom, or the system was refilled recently.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación · Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
- Blocked pumpPossible
The A1 description itself includes a blocked pump. It comes into play when the pressure is between 1 and 1.5 bar, there is no air and A1 keeps appearing.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
Virtual technician
Does A1 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 Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Junkers (Bosch Termotecnia) · 2019 · manual
- 2
Junkers (Bosch Termotecnia) · 2019 · manual
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