- Boilers
- Gas boiler
D3
Junkers d3 on a gas boiler: external controller TB1 tripped
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Junkers d3 means the external temperature controller TB1 has tripped and the boiler stops heating and hot water. Note where the flow temperature dial is set, reset once and, if d3 returns, call the technical service.
Safety first
Stop and check this first
- Do not bridge or disconnect the TB1 controller yourself: it protects the underfloor heating and its connection is inside the boiler. If d3 returns after a reset, leave the heating off and call the technician.
- 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
- Whether you have underfloor heating with TB1
- Flow temperature dial position
- One reset with the reset key
- TB1 and jumper check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: TB1 controller check
- Do not attempt this professional task: 8-9 and PR-P0 jumper check
- Do not attempt this professional task: Flow temperature setting and test
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.
TB1 controller check
What they doThe technician checks the state of controller TB1 and its wiring to the boiler.
They must show youwhether TB1 is closed or open and why.
In writingthe TB1 state and, if it is replaced, the part reference and why.
8-9 and PR-P0 jumper check
What they doOn systems without TB1, the technician checks that the 8-9 and PR-P0 jumpers are fitted on the boiler terminal strip.
They must show youthe terminal strip with the jumpers.
In writingwhat was missing and what was fitted.
Flow temperature setting and test
What they doThe technician reviews the maximum flow temperature set for the underfloor system and tests that the boiler gives heating and hot water without d3.
They must show youthe dial position and the boiler running.
In writingthe temperature set and the test done.
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.
Whether you have underfloor heating with TB1
- Who:
- you.
- How:
- check the installation paperwork or ask your installer whether the boiler heats an underfloor system with a TB1 controller.
- Result that matters:
- whether TB1 exists in your system.
- If there is underfloor heating with TB1:
- d3 points to TB1 having acted; tell the technician.
- If there is no underfloor heating or TB1:
- it points to a missing jumper in the wiring.
- Evidence to keep:
- a photo of the boiler and of any control box near the underfloor manifold.
Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
Flow temperature dial position
- Who:
- you.
- How:
- without touching it, note where the flow temperature dial on the panel is set. Bosch gives position 2, about 48 °C, as an example for underfloor heating.
- Result that matters:
- whether the dial is well above what your installer left set.
- If it is high in a home with underfloor heating:
- tell the technician; it may explain TB1 acting.
- If it is low:
- the setting matters less; it points to TB1 or the wiring.
- Evidence to keep:
- a photo of the dial with the date.
Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
One reset with the reset key
- 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 heating and hot water come back and d3 clears.
- If it works and does not return:
- note the date and time; if it repeats, tell the technical service.
- If d3 returns:
- do not keep trying; Bosch says to contact the official technical service with the fault and the appliance details.
- Evidence to keep:
- a photo of the display with d3 and a note of each reset.
Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
TB1 and jumper check by the technician
- Who:
- authorised technician.
- How:
- checks controller TB1 and its connection at the boiler, or the 8-9 and PR-P0 jumpers if there is no TB1.
- Result that matters:
- whether TB1 acted on temperature or a jumper is missing.
- If TB1 acted:
- they review the flow temperature for the underfloor system.
- If a jumper is missing:
- it is fitted following the Bosch wiring diagram.
- Evidence to keep:
- a report of what was checked and the connection corrected.
The TB1 connection and the jumpers are inside the boiler, at mains voltage: only an authorised technician opens the casing and the terminal strip.
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 d3
- d3
- Error d3 Junkers
- Junkers d3
What it means
In the CerapurComfort fault table, d3 means the external controller has tripped: temperature controller TB1 has acted, or the 8-9 jumper or the PR-P0 jumper is missing. TB1 is a controller wired in on underfloor heating systems connected hydraulically straight to the boiler; when it acts, Bosch states that heating and hot water are interrupted. The code does not tell apart TB1 acting on temperature from a missing jumper in the wiring.
Warnings and stop conditions
- Mediumeditorial warning
Do not bridge or disconnect the TB1 controller yourself: it protects the underfloor heating and its connection is inside the boiler. If d3 returns after a reset, leave the heating off and call the technician.
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
- Underfloor heating controller TB1 trippedPossible
Bosch says temperature controller TB1 has acted. It fits if your home has underfloor heating connected straight to the boiler with a TB1.
Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
- Missing 8-9 or PR-P0 jumperPossible
If your system has no TB1, Bosch states that the 8-9 jumper or the PR-P0 jumper is missing. It fits after a repair or a change to the boiler wiring.
Evidence: Junkers CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
Virtual technician
Does D3 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 CerapurComfort ZWBC 25-2 C, instrucciones de instalación y manejo para el técnico
Junkers (Bosch Termotecnia) · 2012 · manual
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