- Boilers
- Gas boiler
D4
Junkers D4 on a gas boiler: water temperature rising too fast
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Junkers D4 warns that the water temperature is rising too fast or that two sensors disagree. Check the gauge: if it reads below 1 bar, refill to 1 to 1.5 bar, bleed radiators cold at the top and press reset.
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
- Sensors, purge and circulation by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flow and safety sensor comparison
- Do not attempt this professional task: Air purge and pump test
- 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.
Flow and safety sensor comparison
What they doThe technician compares the temperatures read by the flow sensor and the heat exchanger safety sensor, which Bosch says must agree within 15 °C.
They must show youboth readings.
In writingthe values measured against the manufacturer's specification and whether a sensor was replaced.
Air purge and pump test
What they doTurns on purge mode 2.2C and test t3, in which the pump runs for up to 45 seconds and should be heard.
They must show youthe pump running in the test.
In writingthat it was purged and the final pressure.
Information menu reading
What they doReads i1 current status and i2 last error to see which three digit number comes with D4 (341, 342, 213 or 271) and whether it repeats 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 other cause on this page become more likely.
- Evidence to keep:
- a photo of the gauge and of the display showing D4 and its three digit number.
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 D4 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
Sensors, purge and circulation by the technician
- Who:
- authorised technician.
- How:
- reads the cause number, compares the flow and safety sensor readings, purges the boiler with mode 2.2C and runs pump test t3.
- Result that matters:
- whether the sensors agree within 15 °C and whether the water circulates.
- If the sensors disagree with water and no air:
- the sensor or its connection is the suspect.
- If air comes out or the pump does not start:
- the cause is in the circulation, not the sensor.
- Evidence to keep:
- a report with both sensor readings and the tests done.
Bosch reserves removing the casing and any internal work for a competent technician. Sensors, pump and board are inside, 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
- Cerapur D4
- D04
- D 4
- D4 271
- D4 341
- Error D4 Junkers
- Junkers D4
What it means
On the Cerapur Excellence Compact, D4 groups several cause numbers. With 341 the heating flow or return rises too fast and with 342 the hot water temperature does. With 213 and 271 the difference between the flow sensor and the safety sensor exceeds the limit: Bosch says both must agree within 15 °C. For fast rises and excessive differences Bosch names an air lock or too little water. The code does not prove that a sensor is faulty.
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
- Too little water in the systemCommon
Bosch names reduced water content when the temperature rises too fast. You can tell because the gauge reads below 1 bar.
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
An air lock is the other cause Bosch gives for fast rises. It fits if radiators are cold at the top 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)
- Flow and safety sensors that disagreePossible
With 213 or 271 the boiler compares the flow sensor with the safety sensor, which must agree within 15 °C. It comes into play if there is water, no air and D4 continues.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
Virtual technician
Does D4 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
Was this entry useful?