- Boilers
- Gas boiler
8C
Junkers 8C on a gas boiler: super lockout after repeated faults
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Junkers 8C is a super lockout: the boiler stops after too frequent overheating or flame losses. If you smell gas or fumes, leave the home. If not, note the three digit number, press reset once and, if it stays, call the technician.
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.
- If you smell flue gases: Bosch says to shut off the gas supply, open doors and windows, warn the neighbours and leave the building. Carbon monoxide has no smell; if anyone has a headache, dizziness or nausea, get outside and call for help. A CO alarm near the boiler gives early warning.
Safe checks you can perform
- Gas or flue gas smell near the boiler
- Code and three digit number
- Condensate pipe and flue outlet from outside
- A single reset with the reset key
- Flame, combustion and condensate by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Ionisation current and test t6
- Do not attempt this professional task: Combustion analysis
- Do not attempt this professional task: Siphon and flue
- 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.
Ionisation current and test t6
What they doThe technician reads i8 ionisation current with the boiler at maximum and uses test t6 to check the flame detection circuit.
They must show youthe i8 reading.
In writingthe value measured against the manufacturer's specification.
Combustion analysis
What they doConnects a flue gas analyser to the flue test point in chimney sweep mode and measures CO2 and CO at maximum and minimum output.
They must show youthe analyser printout.
In writingthe values measured against the manufacturer's table.
Siphon and flue
What they doCleans the condensate siphon as the manual describes and checks the flue and its terminal.
They must show youthe clean siphon.
In writingthe condition of the siphon and flue.
Information menu reading
What they doReads i1 current status and i2 last error to see which three digit number comes with 8C (373 or 374) 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 note the code.
- 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
Code and three digit number
- Who:
- you.
- How:
- take a photo of the display showing 8C and the three digit number below it, as Bosch asks when you call.
- Result that matters:
- whether the number is 373 or 374.
- If it is 374:
- the clue points to the flame, the condensate or wind in the flue.
- If it is 373:
- the clue points to overheating; tell the technician.
- Evidence to keep:
- a photo of the display, the date, time and outdoor weather.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Condensate pipe and flue outlet from outside
- Who:
- you, only from the ground and if the pipe is at street level.
- How:
- check whether the outdoor condensate pipe has ice or objects in it and whether the flue terminal is damaged or covered. Do not climb ladders.
- Result that matters:
- whether you see ice, a blockage or a damaged terminal.
- If the pipe is frozen:
- thaw it with hot water that is not boiling, as Bosch describes, then hold reset for 5 seconds and wait 2 to 3 minutes.
- If the terminal is damaged or covered:
- do not use the boiler and call the technician.
- Evidence to keep:
- a photo of the pipe and the terminal.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
A single reset with the reset key
- Who:
- you.
- How:
- only if there is no smell of gas, press reset once. Reset appears briefly on the display.
- Result that matters:
- whether the boiler works again or 8C stays on the display.
- If it works:
- note the day and time; if 8C returns, the technician must find the repeating fault.
- If 8C stays:
- Bosch warns that some codes cannot be cleared with a reset; call the service.
- Evidence to keep:
- the reset result.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Flame, combustion and condensate by the technician
- Who:
- authorised gas technician.
- How:
- reads the history, measures the ionisation current, runs a combustion analysis, cleans the siphon and checks the flue.
- Result that matters:
- which fault was repeating: flame or overheating.
- If ionisation is low or the siphon is dirty:
- flame loss is confirmed.
- If the flame is fine:
- the heat exchanger and thermostats become the suspects.
- Evidence to keep:
- a report with the ionisation measured, the analyser printout and the work done.
Burner, electrode, gas valve, siphon and flue are for an authorised gas technician only. Bosch forbids the user from removing the casing.
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
- 8 C
- 8-C
- 8C 373
- 8C 374
- 8C1
- 8C2
- 8E
- 8E1
- 8E2
- Cerapur 8C
- código 8C Samsung
- Error 8C Junkers
- error 8C lavadora Samsung
- Junkers 8C
- Samsung 8C
- Samsung washer 8C
- Samsung washing machine code 8C
What it means
On the Cerapur Excellence Compact, 8C is a super lockout. With number 373 it is caused by too frequent thermostat overheating during operation; with 374, by too frequent ionisation loss, meaning flame loss. For flame loss Bosch names moisture in the sump from a blocked condensate siphon or strong wind in the flue. For overheating it names the main heat exchanger or the flue overheat thermostat. 8C does not point to a part: it sums up a fault that keeps repeating.
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
- Higheditorial warning
If you smell flue gases: Bosch says to shut off the gas supply, open doors and windows, warn the neighbours and leave the building. Carbon monoxide has no smell; if anyone has a headache, dizziness or nausea, get outside and call for help. A CO alarm near the boiler gives early warning.
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
- Repeated flame lossesPossible
This is number 374. Bosch links flame loss to moisture in the sump from a blocked condensate siphon or to strong wind in the flue.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
- Repeated overheatingPossible
This is number 373. Bosch names an overheated main heat exchanger or a tripped flue overheat thermostat.
Evidence: Junkers Cerapur Excellence Compact ZWB...A..., instrucciones de instalación y manejo para el técnico (6 720 820 575)
- Frozen or blocked condensate pipePossible
Bosch warns that a frozen or blocked condensate pipe shuts the boiler down. It fits with 374 after frosty nights if the pipe runs outside.
Evidence: Junkers Cerapur Excellence Compact, instrucciones de uso de la caldera mural a gas de condensación
Virtual technician
Does 8C 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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