Worcester Bosch C4 273 fault codewhat it means and how to fix it
The fan has run continuously for 24 hours, so the boiler has paused firing for two minutes as a precaution. Something has been calling for heat non-stop, and that is what to find.
Before you start
What C4 273 means
Worcester file this under "Fan/Airflow fault" in the Greenstar i table: C4 273, "continuous fan operation for the last 24 hours (appliance temporary blocked)". The Compact family manuals list it among the blocking codes as "airflow present during last 24 hours — fan has been running continuously during the past 24 hours". It is not a fan fault in the sense of C6 or C7. The fan is working; it has simply never been allowed to stop, because for a full day there has been an uninterrupted demand for heat.
Worcester explain that the appliance blocks itself from firing for a minimum of two minutes and then carries on, and the engineer's only check is whether a demand really has been present for the whole 24 hours. On a Greenstar i the number 273 will not be on the display as a lockout; it is a blocking code that you read in the info menu via the spanner button. The same C4 letters carry a second, unrelated meaning in the same table: C4 237 is "control box or HCM defective", an internal error handled like the F0 family.
If the cause code you see is 237 rather than 273, this page is not describing your fault. For 273 the question is why the boiler has been asked to run for a day without pause, and the usual answers are a room thermostat set higher than the house can reach, a programmer or wiring fault holding a permanent demand, or a hot water preheat function that has been left on and keeps the fan cycling.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Room thermostat set beyond what the house can reach | In cold weather a thermostat at 25 °C in a poorly insulated house never satisfies, so the demand is continuous and the fan never stops. | Common |
| Programmer or external control holding a permanent demand | A timer stuck in constant, or a wiring fault putting a live on the demand terminal, keeps the boiler running whatever the room temperature. | Common |
| Radiators that cannot shed the heat | If the boiler is undersized for the property or most radiators are shut, the flow temperature is reached but the room never is, and the demand stays on. | Occasional |
What to try yourself
Look at the room thermostat and the programmer. If the thermostat is set well above the current room temperature, turn it down to a level the house reaches and see whether the boiler now cycles off.
Switch the programmer to off for a few minutes. If the boiler keeps running with no demand from any control, an external wiring fault is holding it on and needs an engineer.
You do not need to reset anything. Worcester describe the block as lasting a minimum of two minutes before the boiler carries on by itself.
Get a Gas Safe registered engineer to look at your Worcester Bosch C4 273
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£180
Time on site
15 minutes
Usually free — a thermostat or programmer setting. Tracing a wiring fault that holds a permanent demand is £90–£180.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
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Related Worcester Bosch fault codes
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: fan and flue
- C7Get it checkedThe fan is not running at all, as distinct from running at the wrong speed. The burner is locked out until it is fixed.Related: fan and flue
- D5Get it checkedThe condensate pump has failed or cannot discharge. Acidic water will back up inside the appliance, so it needs attention promptly.Related: fan and flue
- C1 264Get it checkedAirflow stopped part way through a firing cycle. The fan has quit or the flue is obstructed, and combustion gases may not be leaving safely.Related: fan and flue
- 2910Stop — call an engineerFault in flue system. The boiler has detected a problem with its flue and locked out. Worcester's checks are obstructions, the flue integrity test and water in the sump. Not a code to reset repeatedly.Related: fan and flue
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: same brand
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Related guides
Common questions about Worcester Bosch C4 273
Is the fan about to fail?
Not on this evidence. C4 273 says the fan ran for 24 hours, not that it ran badly. C6 and C7 are the codes for a fan running at the wrong speed or not at all.
It says C4 but the number is 237, not 273.
Then it is an internal control fault rather than this one. Worcester describe C4 237 as the control box or heat control module being defective and treat it like the F0 internal errors — check the HCM is seated, check the control box connections, replace if necessary. That is an engineer visit.
Why would Worcester block the boiler for running too long?
A fan that has never stopped for a day is either serving a genuine constant demand or is a symptom of something holding the boiler on. The two-minute pause costs nothing and gives the control a chance to confirm the demand is real.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.