- Boilers
- Gas boiler
9U
Junkers 9U on a gas boiler: HCM coding plug fault code 233
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
Code 9U (with cause code 233) on Junkers Cerapur gas boilers means the HCM heating control module or coding plug is not recognized (unplugged or wrong plug). Note cause code 233, press reset once, and if 9U remains, call a qualified technician to reseat or replace the HCM plug.
Safety first
Stop and check this first
- Accessing the HCM coding plug requires opening the electronic control unit where 230 V mains terminals are exposed: only a qualified technician may isolate power and open the control box.
- Do not repeatedly reset a safety lockout that will not clear; and if you ever notice a smell of gas, shut off the main gas valve, open windows and doors without touching any electrical switches, vacate the property, and call from outside.
Safe checks you can perform
- Record fault code 9U and three-digit cause code 233
- Check HCM number i22 and version i23 in the read-only info menu
- Single unlock attempt using the reset button
- Inspect the HCM coding plug on the electronic control board
Leave these tasks to a qualified professional
- Do not attempt this professional task: Reseat the HCM coding plug and verify its identification in menu i22
- Do not attempt this professional task: Replace the HCM coding plug if faulty or mismatched
- Do not attempt this professional task: Check the electronic control board and restore service menu settings
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.
Reseat the HCM coding plug and verify its identification in menu i22
What they doWith the electrical supply isolated, the technician locates the HCM coding plug on the electronic board, inspects its pins, seats it firmly, and checks in info menu i22 that the recognized number matches the boiler output and gas type.
They must show youthe seated HCM module on the control board and the three-digit HCM code displayed in parameter i22.
In writingfault code 9U 233, the HCM number confirmed in i22, the reseating performed, and the date.
Replace the HCM coding plug if faulty or mismatched
What they doIf the coding plug is still not recognized after reseating or its number does not match the model table (tables 29 and 47), the technician installs the original HCM coding plug specified for that boiler and gas category.
They must show youthe removed coding plug, the part number of the new original HCM, and the updated i22 and i23 readings.
In writingthe replacement HCM reference fitted, the reason for replacement, and confirmation that 9U cleared.
Check the electronic control board and restore service menu settings
What they doIf reseating or replacing the HCM plug does not resolve the lockout, the technician tests the electronic control board, replaces it if required, and configures the service functions according to the service menu settings sticker.
They must show youthe control board connections and the boiler operating normally with flow temperature on the display.
In writingthe checks carried out on the control board, the service menu parameters set, and the commissioning test result.
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.
Record fault code 9U and three-digit cause code 233
- Who:
- you.
- How:
- check the flashing fault code on the boiler screen alongside the alert triangle or flashing blue LED, and note both the main code (9U) and the three-digit cause code (233) using the spanner/return button if needed.
- Result that matters:
- having both the fault group (9U) and the exact cause code (233) that Bosch/Junkers service requires.
- If the display shows 9U and 233:
- write both down and proceed to check read-only menu i22.
- If a different three-digit number appears under the fault code:
- record the exact pair shown before pressing reset.
- Evidence to keep:
- a photo of the display showing 9U, cause code 233, and the flashing alert indicator.
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) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
Check HCM number i22 and version i23 in the read-only info menu
- Who:
- you, using the external panel buttons without opening any cover.
- How:
- press the spanner/return (Info) button to enter the read-only information menu and use the arrow keys to view i22 (last three digits of the HCM number) and i23 (HCM version number).
- Result that matters:
- verifying without tools whether the control panel can read the HCM coding plug identification.
- If i22 and i23 display values:
- note the digits so the technician can confirm they match your boiler model and gas type.
- If the menu shows no valid HCM number or returns to 9U:
- press the Info button again to exit the menu.
- Evidence to keep:
- the numbers displayed in info parameters i22 and i23, plus the last fault logged in i2.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
Single unlock attempt using the reset button
- Who:
- you.
- How:
- press the reset button on the control panel until Reset briefly appears on the screen (or switch the boiler off and back on once as described in the manual) and wait two to three minutes.
- Result that matters:
- testing whether lockout 9U 233 was caused by a one-off read glitch during initialisation.
- If the unlock succeeds and the flow temperature returns to the screen:
- monitor the boiler and mention 9U at the next service.
- If 9U flashes again and the blue LED keeps blinking:
- do not repeat the reset, book a visit from an official technician.
- Evidence to keep:
- whether Reset appeared on the screen and whether 9U 233 returned immediately.
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) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
Inspect the HCM coding plug on the electronic control board
- Who:
- qualified heating technician or Official Junkers Service.
- How:
- with mains power isolated across all poles, access the HCM coding plug on the control board, check that it is firmly plugged in, and compare its part number against the model table.
- Result that matters:
- determining whether the HCM coding plug was loose, mismatched, or faulty.
- If the HCM plug was loose:
- seat it firmly, restore power, and confirm the correct code appears in menu i22.
- If the plug is damaged or wrong for the appliance:
- fit the correct original HCM plug and, if unresolved, test the PCB.
- Evidence to keep:
- the HCM number verified against the manual table (such as 1614/1615/1618/1619 or 1580-1631) and the i22 readout.
The HCM module plugs into the main control board where 230 V mains voltage is present: never open the boiler casing or control box yourself.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
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
- 9 U
- 9U
- 9U 233
- caldera Junkers 9U
- Cerapur 9U
- Cerapur Excellence Compact 9U
- CerapurComfort 9U
- Junkers 9U
- Junkers 9U 233
- Junkers boiler 9U
What it means
In the technical manuals for Junkers CerapurComfort ZWBE (table 38) and Cerapur Excellence Compact (tables 29 and 33) condensing boilers, lockout code 9U paired with cause code 233 indicates an internal fault with the HCM heating control/configuration module (coding plug not recognized, loose, or incorrect). Because 9U is a class V lockout fault, the heating system shuts down and flashes the code until reset. Seeing 9U 233 on the display does not by itself prove the control board or coding plug is defective before checking its seating and code number.
Warnings and stop conditions
- Highsource verified
Accessing the HCM coding plug requires opening the electronic control unit where 230 V mains terminals are exposed: only a qualified technician may isolate power and open the control box.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
- Highsource verified
Do not repeatedly reset a safety lockout that will not clear; and if you ever notice a smell of gas, shut off the main gas valve, open windows and doors without touching any electrical switches, vacate the property, and call from outside.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
Symptoms and causes
Probable causes · in order
- Loose or improperly seated HCM coding plugCommon
In both CerapurComfort ZWBE and Cerapur Excellence Compact manuals, 9U 233 points first to an HCM coding plug that is not firmly plugged into its socket on the control board.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
- Wrong HCM coding plug for the boiler output or gas typePossible
Each boiler output rating and gas category (natural gas or LPG) requires a specific HCM number (shown in info menu i22); fitting the wrong coding plug triggers 9U 233.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575) · Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
- Defective HCM coding plugPossible
If the coding plug is properly seated and matches the appliance table but is still not recognized by the control unit, the manual notes that the coding plug itself may need replacement.
Evidence: Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
- Faulty socket or main electronic control boardLess common
When the correct HCM coding plug is fitted and replacing it does not clear the lockout, the CerapurComfort service guide instructs checking the electronic control board.
Evidence: Junkers CerapurComfort ZWBE 25-3 C y 30-3 C, instrucciones de instalación y manejo para el técnico (6720847000)
Virtual technician
Does 9U 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
- 3
Junkers (Bosch Termotecnia) · 2015 · manual
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