Worcester Bosch C4 237 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch C4 237 fault code mean?
The C4 fault code on Worcester Bosch boilers signals a fan or airflow anomaly — the boiler's controls have detected that the fan is not behaving as expected, either failing to reach the correct speed, failing to stop when it should, or producing airflow readings that fall outside normal parameters. The sub-code 237 appearing alongside C4 points to an airflow or fan-control irregularity, closely related to the well-documented C4 273 cause code (which specifically flags that abnormal airflow was detected during the previous 24 hours of operation). Because the fan is critical for expelling combustion gases safely through the flue, the boiler locks out immediately to protect occupants. Do not attempt to bypass this lockout.
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
- Fan motor failure or degraded performance Common
The fan motor is the most frequent culprit behind C4 faults. Over time the motor bearings wear, windings develop faults, or dust and debris build up internally causing overheating. When the motor can no longer spin at the speed the control board expects, the boiler shuts down. You may hear an unusual grinding noise, no spin-up sound at all, or a fan that starts briefly then stops.
- Blocked or obstructed flue terminal Common
The external flue outlet can become partially or fully blocked by birds' nests, leaves, ice, snow, or other debris. Even a partial obstruction reduces airflow enough to trigger the C4 fault. This is worth checking visually from outside before calling an engineer, particularly after cold snaps or in autumn.
- Wiring or connector fault between fan and PCB Sometimes
The fan assembly communicates its speed back to the printed circuit board via a tachometer signal wire. If the connector becomes loose, corroded, or the wiring harness is damaged, the PCB receives no speed signal and treats this as a fan failure. This is an engineer-level diagnosis requiring a wiring check and possibly a harness replacement.
- Faulty fan speed sensor or Hall-effect sensor Sometimes
A small sensor on the fan assembly monitors rotational speed. If this sensor fails or drifts out of calibration, the PCB may believe the fan has stopped or is running at the wrong speed even when it is physically spinning correctly. The boiler cannot distinguish this from a genuine fan fault and locks out.
- PCB (control board) fault Rare
If the PCB itself has developed a fault — cracked solder joints, burned tracks, or corrupted firmware — it may misread fan speed data or fail to control the fan relay correctly. PCB faults typically appear alongside other unexplained lockouts or after a power surge. This is a higher-cost diagnosis and should only be confirmed after the fan and wiring have been ruled out.
How to fix it
- Check the external flue terminal for obstructions DIY safe
Go outside and visually inspect the flue outlet on the wall or roof. Look for birds' nests, leaves, ice build-up, or any debris partially blocking the terminal. If you can safely reach it, clear any loose debris by hand. Do not attempt to modify or re-route the flue pipe itself — that is Gas Safe work.
- Verify the gas supply is working DIY safe
Check that other gas appliances in your home (cooker, gas fire) are working normally, and confirm your gas meter is on. If you have a prepayment meter, ensure it has sufficient credit. A loss of gas supply can occasionally cause fan-related lockouts as the boiler attempts to fire unsuccessfully.
- Reset the boiler once or twice DIY safe
Press and hold the reset button (usually marked with a flame symbol or 'Reset') for around 3 seconds, then release. Allow the boiler up to 2 minutes to complete its start-up sequence. Listen carefully — you should hear the fan spin up before ignition. If the boiler locks out again within a short time, do not keep resetting; repeated resets on a combustion-related fault can mask a safety issue and are not a fix.
- Check system pressure is within range DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 0.5 bar, top up via the filling loop. Very low pressure can contribute to boiler faults, though it is unlikely to be the sole cause of a C4 fault.
- Call a Gas Safe registered engineer to diagnose the fan and airflow system Gas Safe engineer
If the fault persists after a reset, you need a Gas Safe engineer. They will test the fan motor, check the tachometer output and wiring, inspect the flue system for leaks or blockages, and use diagnostic software to read the full fault history. Do not attempt to remove or test the fan assembly yourself — this requires working inside the boiler casing and on components that must only be handled by a qualified engineer.
- Engineer to replace or repair the faulty component Gas Safe engineer
Once the root cause is identified, the engineer will carry out the appropriate repair: replacing a worn fan motor assembly, renewing faulty wiring or connectors, replacing a defective speed sensor, or — if all other components test correctly — replacing the PCB. Always ask for a parts warranty and check that any replacement fan is the correct specification for your boiler model.
Parts you may need
- Boiler fan assembly (motor and impeller) · from £155
- Fan speed sensor / Hall-effect sensor · from £35
- Fan wiring harness and connectors · from £25
- Printed circuit board (PCB) · from £280
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 £220–£420, depending on the underlying cause.
Frequently asked questions
Is C4 237 the same as C4 273 on Worcester Bosch boilers?
They are closely related. Worcester Bosch fault codes have two parts: a letter-number fault code (C4) and a numeric cause code (237 or 273). C4 273 is the most widely documented sub-code and specifically indicates that abnormal airflow was detected in the previous 24 hours. C4 237 points to a similar fan or airflow control anomaly. Both fall under the same C4 fault family and are diagnosed and repaired in the same way — the troubleshooting steps on this page apply to both.
Can I keep resetting a C4 fault to get hot water while I wait for an engineer?
You can try one or two resets to see if the fault clears, but you should not keep resetting a boiler that is locking out on a fan or airflow fault. The C4 code means the boiler has detected a problem with combustion airflow — repeatedly forcing restarts on a fault like this can mask a genuine safety issue. If the boiler locks out again after a reset, leave it off and arrange for a Gas Safe engineer as soon as possible.
How much does it cost to fix a C4 fault on a Worcester Bosch boiler?
For the most common repair — replacing the fan assembly — most homeowners in the UK pay between £220 and £420 including parts and labour. A straightforward fan replacement at the lower end can cost around £225–£300 if the engineer diagnoses it on the first visit. If the fault turns out to be a PCB issue, costs typically rise to £350–£550 including parts. In rare cases where multiple components need replacing, the bill could exceed £600, at which point it is worth discussing whether a new boiler makes more financial sense, particularly if your current boiler is over 10–12 years old.
What should I do if my external flue terminal is blocked with ice or a bird's nest?
If you can see and safely reach the flue terminal from ground level, you can carefully remove loose debris such as leaves or a nest with your hand or a soft brush. Never use tools that could damage the terminal, and never attempt to modify the flue pipe itself. If ice is blocking the terminal in freezing weather, allow it to thaw naturally or gently pour warm (not boiling) water over the terminal. Once clear, reset the boiler. If the blockage is at height or inside the flue duct, a Gas Safe engineer must deal with it.