- Boilers
- Gas boiler
C6
Worcester C6 on a gas boiler: fan speed too low
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Greenstar CDi boilers, C6 means the fan speed is too low, so the boiler cannot run safely. Check from outside that the flue terminal is not blocked, reset once for 3 seconds and, if C6 returns, call a Gas Safe registered engineer.
Safety first
Stop and check this first
- This code involves the flame or the flue, so check that a working carbon monoxide alarm is fitted, as HSE strongly recommends. If it sounds, switch the boiler off, open windows, get everyone outside and call 0800 111 999.
- If you smell gas: do not smoke or strike matches, do not operate electrical switches or unplug anything, do not use the phone or ring doorbells. Turn off the gas at the meter, open windows and doors, warn your neighbours and leave. Well away from the building, call the National Gas Emergency Service on 0800 111 999.
Safe checks you can perform
- Flue terminal seen from outside
- When C6 appears
- One reset
- Fan circuit test
Leave these tasks to a qualified professional
- Do not attempt this professional task: Supply voltage and fan lead test
- Do not attempt this professional task: Fan or PCB replacement only after tests
- Do not attempt this professional task: Combustion check after fan work
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.
Supply voltage and fan lead test
What they doWith all household appliances on, the engineer measures the supply voltage, then checks continuity of each core of the fan lead with the boiler isolated.
They must show youthe voltage reading and the continuity test.
In writingthe measured values and any lead or plug repaired.
Fan or PCB replacement only after tests
What they doIf wiring and supply are correct, the engineer replaces the fan and, if C6 continues, the PCB, noting and restoring service settings.
They must show youthe boiler running without C6 after the change.
In writingthe part replaced, its reference and why.
Combustion check after fan work
What they doAfter work on the fan, the engineer measures the flue gas CO2 level with an analyser and compares it with the installation instructions.
They must show youthe analyser reading.
In writingthe measured values against Worcester's figures.
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.
Flue terminal seen from outside
- Who:
- you, from the ground.
- How:
- look at the flue terminal outside without touching or climbing; Worcester says not to operate the boiler if the terminal is obstructed or damaged.
- Result that matters:
- whether anything blocks or damages the terminal.
- If it is blocked or damaged:
- leave the boiler off and tell the engineer.
- If it looks clear:
- the fan or its supply are the main suspects.
- Evidence to keep:
- a photo of the terminal taken from the ground.
Evidence: Worcester Greenstar CDi Classic combi user instructions 6 720 812 179
When C6 appears
- Who:
- you.
- How:
- note whether C6 shows when lots of electrical appliances are on (oven, kettle, washing machine).
- Result that matters:
- whether it links to electrical load.
- If it appears only under heavy electrical load:
- tell the engineer; supply voltage is checked first.
- If it appears regardless:
- the fan circuit becomes the focus.
- Evidence to keep:
- a note of time and appliances running.
Evidence: Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
One reset
- Who:
- you.
- How:
- press the reset button for three seconds.
- Result that matters:
- whether C6 stays away.
- If C6 returns:
- leave the boiler off and call a Gas Safe registered engineer; Worcester says not to rectify listed faults yourself.
- If it clears:
- note it; a repeat confirms an intermittent fault.
- Evidence to keep:
- a photo of the display and a note of each reset.
Evidence: Worcester Greenstar CDi Classic combi user instructions 6 720 812 179
Fan circuit test
- Who:
- a Gas Safe registered engineer.
- How:
- checks the fan connector, measures supply voltage and fan lead continuity, then tests the fan and PCB.
- Result that matters:
- which step of the booklet finds the fault.
- If the lead or plug fails:
- it is reconnected or replaced, not the fan.
- If the lead tests correct:
- the fan or PCB is replaced.
- Evidence to keep:
- a job sheet with the measurements.
The fan sits in the sealed combustion circuit with mains wiring; only a Gas Safe registered engineer opens it.
Evidence: Worcester Greenstar CDi Classic combi user instructions 6 720 812 179 · Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
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
- Worcester Bosch
- Product type
- Gas boiler
Known code variants
- AEG asciugatrice C6
- AEG C6 error
- AEG dryer C6
- AEG Fehler C6
- AEG secadora C6
- AEG sèche-linge C6
- AEG Trockner C6
- Bosch C6 caldera
- C06
- C-6
- C6 AEG asciugatrice
- C6 AEG dryer
- C6 AEG secadora
- C6 AEG sèche-linge
- C6 AEG Trockner
- C6 Worcester
- Cerapur C6
- code erreur AEG C6
- error AEG C6
- Error C6 Junkers
- errore AEG C6
- Greenstar C6
- Junkers C6
- Worcester boiler C6 fault
- Worcester Bosch C6
- Worcester C6
What it means
In Worcester's Greenstar CDi Classic tables, C6 is fan speed too low. The fan pushes combustion products into the flue, and the control stops the boiler when the speed it reads is under target. The service booklet checks, in order, the fan cable connector, the mains supply voltage with all household appliances on, the continuity of the fan lead, the fan itself and finally the PCB. The code does not single out one of these parts.
Warnings and stop conditions
- Higheditorial warning
This code involves the flame or the flue, so check that a working carbon monoxide alarm is fitted, as HSE strongly recommends. If it sounds, switch the boiler off, open windows, get everyone outside and call 0800 111 999.
Evidence: HSE gas safety for homeowners · Worcester Greenstar CDi Classic combi user instructions 6 720 812 179
- Criticalsource verified
If you smell gas: do not smoke or strike matches, do not operate electrical switches or unplug anything, do not use the phone or ring doorbells. Turn off the gas at the meter, open windows and doors, warn your neighbours and leave. Well away from the building, call the National Gas Emergency Service on 0800 111 999.
Evidence: Worcester Greenstar CDi Classic combi user instructions 6 720 812 179
Symptoms and causes
Probable causes · in order
- Fan connector or leadPossible
Fits if C6 started after work inside the boiler or comes and goes; the booklet checks the plug on the fan and the continuity of each core first.
Evidence: Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
- Low mains voltagePossible
Fits if C6 appears when many electrical appliances are running; the engineer measures the supply with them all on.
Evidence: Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
- Fan or control board faultPossible
Fits when wiring and supply test correct and C6 persists; the booklet then replaces the fan and, last, the PCB.
Evidence: Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
Virtual technician
Does C6 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
Health and Safety Executive · official_doc
- 2
Worcester Greenstar CDi Classic combi user instructions 6 720 812 179
Worcester Bosch · 2014 · manual
- 3
Worcester Greenstar CDi Classic engineer's service booklet 6 720 830 033
Worcester Bosch · 2014 · manual
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