Worcester Bosch C4 273 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch C4 273 fault code mean?
The C4 273 fault code indicates that your Worcester Bosch boiler has detected continuous fan or airflow activity over the preceding 24 hours. As a safety measure, the boiler shuts itself down — typically for around two minutes — before locking out. In plain terms, the appliance has noticed the fan has been running without a proper rest cycle, or that airflow is present when the boiler expects it to have stopped. This points to a problem either with the fan itself, the components that control it, or with how flue gases are being expelled. The boiler will not resume normal operation until the root cause is identified and resolved. Some engineers refer to the C4 family of codes more broadly as fan and flue-related faults on Worcester Bosch Greenstar appliances — if you see C4 displayed alongside 273 on your screen, this is the specific sub-code you are dealing with, and it relates directly to prolonged or unexpected airflow detection.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty fan motor or fan speed sensor Common
The fan motor or its speed-sensing component may be running continuously or failing to signal a proper stop to the boiler's control board. After each heating cycle the fan should run briefly to purge residual flue gases, then stop — if it keeps running beyond that, the C4 273 lockout is triggered. A worn motor, a damaged speed sensor, or degraded wiring to the fan assembly are the most frequently diagnosed causes of this code.
- PCB or fan relay fault Common
The printed circuit board governs when the fan starts and stops. A faulty fan relay on the PCB, or a broader PCB fault, can cause the board to keep signalling the fan to run indefinitely. Engineers often check the fan relay and associated timing circuits early in the diagnostic process when this code appears.
- Flue or terminal blockage Common
A partial or full obstruction in the flue system — caused by debris, a bird's nest, ice during cold snaps, or a displaced flue terminal — can cause unusual back-pressure or airflow readings that confuse the boiler's controls and lead to extended fan activity. External flue terminals are worth checking visually as a first step.
- Heat exchanger temperature sensor fault Sometimes
If the sensor monitoring heat exchanger or hot water temperature is defective or has a poor electrical connection, the boiler may not correctly detect that it has fully cooled down after a cycle. This can cause the fan to keep running as the appliance waits for a temperature threshold that the sensor never reports reaching.
- High-limit thermostat activation Sometimes
The high-limit thermostat is a safety cut-out that activates when the boiler detects dangerously high temperatures — often caused by poor water circulation or a pump issue. Repeated or sustained activation can result in extended fan run times as the boiler attempts to cool down, ultimately triggering the C4 273 code.
- Pressure sensor or low system pressure Sometimes
A defective pressure sensor, or genuinely low system pressure due to a leak or recent bleeding of radiators, can interfere with normal boiler operation and indirectly contribute to fan-related fault codes. The system should read between 1 and 1.5 bar when cold.
- Burner control unit or code plug fault Rare
On some Greenstar models, the burner control unit or code plug — a small component that tells the boiler its own specification — can develop faults or become mismatched. This can cause software-level errors that manifest as unusual fan behaviour and trigger C4 codes.
How to fix it
- Check the external flue terminal for obvious obstructions DIY safe
Go outside and look at where the flue exits the building. Check for visible blockages such as debris, a bird's nest, or ice covering the terminal grille. Do not attempt to dismantle or modify the flue — a visual inspection only. If you see an obvious blockage you can safely clear from the outside (such as loose leaves), remove it carefully.
- Verify your gas supply is working DIY safe
Try another gas appliance in your home — a gas hob, for example. If those are also not working, the issue may be with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 if you suspect a supply interruption or smell gas.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, top it up using the filling loop — refer to your boiler's user manual for the correct procedure for your model. Do not overfill beyond 2 bar.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (marked with a flame symbol on most Greenstar models) for approximately three seconds until the display changes. Allow the boiler a few minutes to attempt a restart. If it fires up and runs normally, monitor it over the next 24–48 hours. If the C4 273 code returns, do not reset again — repeated resets will not fix an underlying fault and could mask the problem from an engineer.
- Do not attempt further investigation of fan, PCB, or sensor components Gas Safe engineer
The fan assembly, wiring, PCB, sensors, and flue system connections must only be inspected and worked on by a Gas Safe registered engineer. Attempting to access these components yourself is unsafe, may invalidate your boiler's warranty, and is not permitted under UK gas safety regulations.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will test the fan motor and speed sensor, inspect the flue system for blockages or pressure issues, check the PCB and fan relay, and test temperature sensors and the high-limit thermostat. They will carry out any necessary repairs and perform a safe reset to clear the fault code. Always ask to see the engineer's Gas Safe ID card before work begins — you can verify registration at gassaferegister.co.uk.
Parts you may need
- Fan assembly (Worcester Bosch Greenstar compatible) · from £175
- Fan speed sensor / tachometer · from £35
- Printed circuit board (PCB) · from £220
- High-limit thermostat · from £25
- NTC heat exchanger temperature sensor · from £20
- Pressure sensor / transducer · from £30
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£380, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my boiler to clear the C4 273 code?
A single reset is worth trying, but if the fault returns you should stop resetting and call an engineer. The C4 273 code means the boiler has been running its fan continuously for 24 hours — there is an underlying fault causing this, and repeated resets will not fix it. They can also make it harder for an engineer to diagnose the problem, as they may not be able to reproduce the fault condition when they arrive.
How much does it typically cost to fix a C4 273 fault on a Worcester Bosch boiler?
Most people with this fault pay somewhere between £150 and £380, depending on what the engineer finds. A fan speed sensor or thermostat replacement at the lower end, or a new fan assembly with labour in the mid-range, will fall within that bracket. A PCB replacement tends to be at the upper end or slightly beyond it — typically £280 to £420 including labour. In rare cases where a heat exchanger is at fault, costs can exceed £500, but that outcome is uncommon for a C4 273 specifically.
What is the difference between the C4 code and C4 273 on my Worcester Bosch boiler?
Worcester Bosch displays fault codes as a two-part combination — a letter-number prefix (such as C4) followed by a three-digit sub-code (such as 273). The C4 prefix broadly covers fan and flue-related faults, while the 273 sub-code narrows it down to continuous fan or airflow detection over a 24-hour period. If your display shows just C4, or shows C4 with a different number, the cause may be slightly different — but the diagnostic process for fan and flue faults is similar across the C4 family.
My Worcester Bosch boiler is over 10 years old and keeps showing fault codes — should I repair or replace it?
If your boiler is between 10 and 12 years old and has required more than one significant repair in the past couple of years, it is worth getting an honest assessment from your engineer about its remaining lifespan. A new Worcester Bosch Greenstar combi typically costs between £1,400 and £2,000 installed, which can represent better value than ongoing repairs on an ageing appliance — particularly as older boilers are also less energy-efficient than current A-rated models. Worcester Bosch's own Repair Care Plan covers boilers under 12 years old and may be worth exploring if your boiler is still within that window.