Worcester Bosch C1 fault code: the fan stopped during operation
C1 carries the cause code 264, so you will see the same fault written as C1, as C1 264, or as a bare 264 read from the boiler's info menu. It means the fan stopped while the boiler was running. Here is what it means, whether it is dangerous, what you can safely check, and when to call an engineer.
What does the Worcester Bosch C1 fault code mean?
The Worcester Bosch C1 fault code means the fan stopped while the boiler was running, so the boiler shut itself down. Worcester Bosch groups it under Fan/Airflow fault with cause code 264. The installation and servicing instructions for the Greenstar i ErP and the Greenstar i System ErP word it as "Fan stopped during operation". The instructions for the Greenstar i System Compact ErP word the same C1 264 row as "Airflow stopped during operation", with the possible cause given as "Fan has stopped".
The fan drives the products of combustion out through the flue. Once the control board can no longer prove the fan is turning, it stops the boiler rather than let it keep burning gas without a proven path for the flue gases. The shutdown is a protection, and it should never be worked around.
C1 is the code group shown on the display and 264 is the three-digit cause code stored behind it. Worcester Bosch's instructions describe the cause code as being read from the info menu using the info and return button on the boiler fascia, the one marked with a spanner, and 264 is the number an engineer will ask you for. The Greenstar i System Compact ErP instructions list C1 264 as a blocking error rather than a locking error: the boiler stops without flashing fault data at you, and the code is read from that menu instead.
There is a reason a search for a Worcester C1 fault turns up so little from Worcester Bosch itself. Checked in August 2026, we could not find C1 explained anywhere on Worcester Bosch's consumer support pages, and the online troubleshooting tool is a questionnaire with no code list. The meaning of C1 264 is published inside the installation and servicing instructions written for installers, which anyone can download free from the Worcester Bosch manuals and brochures library.
Key facts
The quick version, before the detail.
- Meaning: the fan stopped while the boiler was running. Worcester Bosch words it as "Fan stopped during operation".
- Cause code: 264. In the Greenstar i ErP, Greenstar i System ErP and Greenstar i System Compact ErP instructions, C1 carries this one cause code, so C1 and C1 264 are the same fault.
- Is it dangerous? Not on its own. The boiler stopping is the protection doing its job, and it is safe to leave the boiler switched off until an engineer attends.
- Fault type: the Greenstar i System Compact ErP instructions list C1 264 as a blocking error, which clears by itself once the fan runs again, rather than a locking error that flashes a warning triangle and needs a reset.
- Where it is published: in the installation and servicing instructions written for installers. Checked in August 2026, Worcester Bosch does not explain C1 on its consumer support pages.
- DIY fixable? Not the fault itself. The fan, its wiring and the control board all sit inside the casing, so your safe checks are the pressure gauge, the electrical supply, a ground-level look at the flue terminal, and one single reset.
- Ranges checked against the manuals: Greenstar i ErP combi (Greenstar 25i ErP and 30i ErP), Greenstar i System ErP and Greenstar i System Compact ErP, all digital-display boilers. The Greenstar CDi Classic uses a code set with no C1 in it and shows fan faults as C6, and the Greenstar Ri ErP+ has no digital display and signals faults by LED flash patterns. Other Greenstar ranges were not checked, so a C1 on one of those is not ruled in or out here.
Is the C1 fault dangerous, and is it an emergency?
A C1 code on its own is not a gas emergency. The boiler has stopped because it can no longer prove the fan is clearing combustion gases through the flue, and stopping is exactly what it is designed to do. It is safe to leave the boiler switched off until a Gas Safe registered engineer attends, and you should leave it off if C1 keeps returning, or if you heard any rattling, grinding or laboured noise before it stopped.
The National Gas Emergency Service line, 0800 111 999, is for when you can smell gas or your carbon monoxide alarm is sounding. A C1 code by itself is not a reason to call it. If you do smell gas, or the alarm goes off, leave the property and call 0800 111 999 from outside. Do not touch switches on your way out, and do not go back in until you are told it is safe.
What you must never do is force the boiler to keep running. The fan is what proves the flue path is clear, so overriding the shutdown removes the protection that stops the boiler burning gas with nowhere safe for the flue gases to go.
Why does the C1 264 fault code appear?
C1 points at the fan and its electrical supply, not at the gas valve or the flame. The check list Worcester Bosch prints against the C1 264 row is short: inspect the fan harness and connector, confirm the control board is supplying the fan, and test the fan itself, replacing it if necessary. The rows either side add a check of the control box connections, and replacement of the control box if needed. All of that is engineer work behind the casing.
In practice that means a failed fan or fan motor, a loose or corroded connector, damaged wiring, a control board no longer powering the fan, or something obstructing the fan or the air path. A restricted flue or air path belongs on that list too. Worcester Bosch does not name it on the C1 264 row itself, but it is a genuine cause an engineer has to rule out, and it is the one with a carbon monoxide risk attached, so it should never be skipped. Third-party lists also add a failed air pressure switch, which is not on the C1 row either, so treat that as something an engineer may check rather than the manufacturer's stated cause.
It is worth separating C1 from the fan codes printed next to it in the same table, because third-party lists mix them up constantly. In the Greenstar i ErP and Greenstar i System ErP instructions the Fan/Airflow rows read: C7 214, fan does not run during the start-up phase; C6 215, fan speed too high; C6 216, fan speed too low; C7 217, fan has stopped during appliance operation; and C1 264, fan stopped during operation. C1 264 and C7 217 mean almost the same thing, so do not read a sharp difference into them: which one appears depends on the model and on how the control board logged it. Anything you read saying C1 means low water pressure is wrong, and anything saying C1 means fan speed too low is describing C6 216.
C1 264 causes and what to do about each one
| Likely cause | What you may notice | What to do |
|---|---|---|
| Failed fan or fan motor | C1 returns almost straight away, and nothing spins up when the boiler tries to light. | Engineer only. The fan is tested and replaced from behind the casing. |
| Loose, corroded or damaged fan harness or connector | The fault comes and goes, often after other work on the boiler. | Engineer only. The harness and connector are inspected, reseated or renewed. |
| The control board no longer supplying the fan | The fan never runs, even though it is undamaged. | Engineer only. The supply is measured at the fan and the control box connections are checked. |
| An obstruction stalling the fan or restricting the air path | A rattling, grinding or laboured noise before the boiler stops. | Engineer only behind the casing. Outside, once the boiler is off and the terminal has cooled, you may clear loose leaves or snow from around the flue terminal. |
| A restricted or blocked flue | The fault repeats in bad weather, or after leaves, snow or a nest build up at the terminal. | Engineer only, and never skipped. Flue inspection and combustion testing are Gas Safe registered work, and this is the cause that carries a carbon monoxide risk. |
What you can safely check yourself
You can rule a few things in or out from outside the boiler, and what you find is useful to the engineer who attends. Every step below is done with the casing closed.
1. Look at the pressure gauge and note the reading. Between 1 and 1.5 bar with the system cold is normal. C1 is not a pressure code, but the reading is worth having.
2. Check the fused spur or switch feeding the boiler is on, and that nothing has tripped at the consumer unit.
3. Switch the boiler off and let the flue terminal cool, then look at it from ground level outside. Look only. Clear loose leaves or snow from around the terminal, never reach into it, never push anything into it, and never use a tool or a ladder. The terminal discharges hot flue gas and condensate and can burn you. Report anything else you find, such as a nest, packaging or damage, to the engineer instead of touching it.
4. Switch the boiler back on and listen for the fan. When the boiler tries to light you should hear it whirring for a few seconds before the burner fires. Silence is worth reporting.
5. If the boiler has not already restarted itself, press the reset button once, and treat that as a test rather than a fix. If C1 comes back, or if you heard any rattling, grinding or laboured noise, switch the boiler off and book a Gas Safe registered engineer. It is safe to leave the boiler switched off while you wait.
There are things you must not do. Do not open the boiler casing, touch the fan, its wiring or the control board, test the electrical supply yourself, or use a ladder to reach a flue terminal at height. And do not press reset over and over to force the boiler to run: the Greenstar i System Compact ErP instructions class C1 264 as a blocking error that clears on its own once the fan runs again, so repeated resetting only hides a fault that is still there.
When to call a Gas Safe registered engineer
Call a Gas Safe registered engineer if C1 returns after a single reset, if you cannot hear the fan at all, or if the boiler keeps stopping and starting. The diagnosis follows Worcester Bosch's own list: inspect the fan harness and connector, confirm the control board is supplying the fan, test the fan and replace it if necessary, then check the control box connections and replace the control box if that is at fault. A restricted flue or air path is ruled out at the same visit.
A new fan or control board costs more than a reset, so it is worth deciding how you would rather cover it. Book a one-off repair with us, or call 0333 772 6247. A Smart Plan boiler and central heating module covers parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it is over 7, and you pay anything above that limit yourself. Smart Plan is a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. It comes from UK Boiler Company Ltd, trading since 2014.
The small print: cover applies from day one, and a £95 call-out fee, paid in advance, applies to visits in your first 30 days, to faults the plan does not cover, to no-access visits and to early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00 unless it is a genuine emergency breakdown, and we aim to get an engineer out within two working days. Using a module starts a 12-month agreement period from your first service, with a 14-day cooling-off period that ends once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.
Worcester Bosch C1 fault code FAQs
What does C1 mean on a Worcester Bosch boiler?
C1 means the fan stopped while the boiler was running, so the boiler shut down. Worcester Bosch records it as a Fan/Airflow fault with cause code 264 and words it as "Fan stopped during operation", which is why the same fault is often written C1 264, or simply 264.
Is C1 the same fault as C1 264?
Yes. C1 is the code group on the display and 264 is the cause code stored behind it, read from the boiler's info menu, so C1, C1 264 and a bare 264 all describe the same fault: the fan stopping while the boiler was running.
Is a Worcester Bosch C1 fault dangerous?
Not on its own. The boiler has shut itself down because it can no longer prove the fan is clearing combustion gases, and that shutdown is the protection working. It is safe to leave the boiler switched off until a Gas Safe registered engineer attends. Call the National Gas Emergency Service on 0800 111 999 only if you can smell gas or your carbon monoxide alarm is sounding, and make that call from outside the property.
Why can I not find C1 on the Worcester Bosch website?
Because it is not published on the consumer pages. Checked in August 2026, the consumer FAQ on error symbols lists display symbols and the SE service code, and the troubleshooting tool is a questionnaire with no code list. C1 264 appears in the installation and servicing instructions written for installers, which are free to download from the Worcester Bosch manuals and brochures library.
What is the difference between C1, C6 and C7?
They are all Fan/Airflow rows in the same manufacturer table, but they log different things. In the Greenstar i ErP and Greenstar i System ErP instructions, C7 214 is the fan not running during the start-up phase and C7 217 is the fan stopping during appliance operation. C6 215 is fan speed too high and C6 216 is fan speed too low. C1 264 is the fan stopping during operation, so it sits very close to C7 217, and which one appears depends on the model and on how the control board logged it.
Can I fix a Worcester C1 fault myself?
There is no safe do-it-yourself fix. You can check the pressure gauge and the electrical supply, look at the outside flue terminal from ground level with the boiler switched off, listen for the fan and reset once. Testing the fan, its harness or its electrical supply means opening the casing, which is work for a Gas Safe registered engineer.
Will resetting clear a C1 264 fault for good?
Only if it was a one-off. The Greenstar i System Compact ErP instructions class C1 264 as a blocking error, which clears on its own once the underlying fault clears. A boiler that keeps returning to C1 has a genuine fan, wiring, control board or flue problem, so resetting again and again is not the answer.
Which Worcester Bosch boilers show C1?
C1 264 appears in the installation and servicing instructions for the Greenstar i ErP combi range (Greenstar 25i ErP and 30i ErP), the Greenstar i System ErP range and the Greenstar i System Compact ErP range, all digital-display boilers. The Greenstar CDi Classic uses a code set that has no C1 in it and shows fan faults as C6, and the Greenstar Ri ErP+ has no digital display and signals faults by LED flash patterns. Other Greenstar ranges were not checked against their manuals, so check yours before assuming.
Seeing C1 or C1 264? We can get an engineer out to you.
Book a one-off repair and a Gas Safe registered engineer will test the fan, its harness and its supply. Or set up a Smart Plan boiler and central heating module, which covers parts and labour up to £500 a year for a boiler under 7 years old and up to £200 a year if it is older. It is a service plan, not insurance. Cover applies from day one, though a £95 call-out fee, paid in advance, applies to visits in your first 30 days, and to faults the plan does not cover.
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