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Want to make your hot air heating system more energy efficient? This guide covers what homeowners need to know about boosting energy efficiency in hot air heating systems: practical tips, maintenance advice, and upgrade options to help you save money and reduce your carbon footprint. At 0800 Homefix, we specialise in optimising heating systems to ensure they deliver maximum efficiency and comfort for your home.
If your boiler is displaying the Worcester C4 error, it’s alerting you to a fan or flue fault-both of which are critical to safe and efficient boiler operation. This is one of several boiler error codes used by Worcester Bosch to diagnose faults in the system. At 0800 Homefix, we’ve handled countless C4 faults, particularly on models like the CDI Classic Combi Boiler, a premium-range, high-performance model designed for homes with high hot water demand, and the Greenstar Si Compact.
The Bosch CDI Classic Regular is another model where C4 and other fault codes are common; this discontinued, high-capacity boiler was designed for larger homes or properties with higher hot water requirements. The Bosch Greenstar Si Compact stands out for its compact design, making it ideal for installation in small spaces such as kitchen or airing cupboards, and C4 as well as other error codes are also frequently encountered on this model.
This in-depth guide explains everything from causes and symptoms to key components like the heat exchanger, temperature sensor, gas valve, and pressure sensor that may be involved.
What Is the Worcester Bosch Boiler Error C4 Code?
The C4 on Worcester Bosch boilers indicates the fan isn’t working correctly or the flue gases aren’t being expelled properly. This is a Worcester Bosch boiler error code used to diagnose specific faults. Since these systems manage combustion safety, any failure leads to a full shutdown.
Your display might show:
Worcester boiler C4 fault
Worcester Bosch showing C4
Combi boiler error codes C4
This fault can be triggered by electrical issues, sensor misreadings, or physical blockages affecting components like the flue gas system, burner control unit, or fan motor.
Issues with the central heating CH flow sensor, such as a flow negative thermistor coefficient or a temperature sensor defective, can also cause the C4 error. Additionally, a pressure sensor defective or a faulty outside temperature sensor may also trigger the C4 code.
Worcester Fan or Flue Issue – Key Symptoms
If you’re seeing the C4 fault, you may also notice:
A complete stop in heating or hot water
No sound from the ignition burner control unit
An attempt to start followed by shutdown
LED display stuck on heating mode with no progress
Occasional FA flame detected warning
The issue may be linked to the flow and safety sensor, air-gas ratio control, or gas valve relay.
Components Related to Worcester C4 Faults and Heat Exchanger Temperature Sensor
Understanding the parts behind the error helps with diagnosis. The following components can trigger or be affected by the C4 error:
Fan Motor – responsible for pushing exhaust gases through the flue
Flue Vent System – includes external terminals that may be blocked
Flow Temperature Sensor – if the temperature sensor is defective, it can trigger false readings. Ensure the flow temperature sensor is installed and connected correctly to avoid misdiagnosis.
Gas Valve & Gas Valve Relay – controls the flow of fuel; issues may arise with modulating gas valve, minimum burner load, or gas valve EV1 leak
Heat Exchanger Temperature Sensor – poor circulation or sensor faults can cause a C4 code
Negative Thermistor Coefficient (NTC) Sensors – used in flow and hot water temperature sensors. The hot water temperature sensor detects the temperature of hot water; if it is defective or improperly connected, it can cause error codes and may require replacement or correct installation.
Pressure Sensor – if defective or has poor electrical connector contacts, it can block system startup
Control Unit / Burner Control Unit – software errors like incorrect code plug detected, incompatible code plug, or code plug not detected are possible triggers. Always check for the correct code plug when troubleshooting these errors to ensure proper communication between the control unit and the boiler system.
High Limit Thermostat – a safety device that triggers if the system overheats, preventing excessive temperature and potential damage.
Safety Temperature Limiter – when tripped, can cause flow temperature rising beyond safe limits
Causes of Worcester Boiler C4 Fault
Some of the common reasons we diagnose include:
Sensor short circuit or damaged sensor port
Blocked or incorrectly installed flue system
Burner operation faults and flame signal loss
Fault occurs repeatedly due to incorrect part configuration or damaged cable exchange fuse
Ev1 leak test failed during burner start-up
Ev2 leak test failed, valve ev1 leak test, or valve ev2 leak test indicating gas valve or leak detection issues
Incorrect installation position of the fan or sensor
System pressure drop or pressure sensor connection issues
Need to check pressure sensor connection as part of diagnosing C4 faults
Central heating boil detected, especially when operating at minimum burner load with a large temperature difference between flow and return
Abnormal flow temperature rise within the heat exchanger
This fault may also stem from issues in the programmer interface module, bus communication cable, or incompatibility in the interconnected room air supply.
How to Troubleshoot C4 on Worcester Bosch Boiler
Reset the Boiler: Clear minor electrical faults and listen for the fan starting.
Check the Fan Function: No sound or failed spin-up? Likely a mechanical or electrical issue.
Inspect External Flue: Obstructions like leaves, snow, or nests may impact airflow.
Monitor System Pressure: Low central heating system pressure or pressure sensor replace required.
Call 0800 Homefix: If none of these resolve it, contact professionals.
⚠️ This error affects combustion and fuel supply. DIY repair beyond basic checks is not safe.
When to Contact 0800 Homefix
At 0800 Homefix, we handle Worcester Bosch boiler errors daily, including the full range of bosch boiler error codes.
We offer:
Fan motor testing and replacements
Pressure and flow sensor diagnostics
Gas valve leak repairs
Control unit and code plug fault resolution
Fast, same-day service for urgent boiler lockouts
All work includes safety checks, system resets, and we only use genuine manufacturer-approved parts.
Repair Costs for Worcester Bosch C4 Error
Fan replacement: £150–£280
Pressure or temperature sensor: £90–£180
Wiring or software-related repairs: £100–£250
Flue system cleaning or replacement: £80–£160
We provide clear pricing, fixed quotes, and no hidden costs.
Preventing Future C4 Errors
Annual servicing for all Worcester Bosch boilers
Check for temperature difference greater than 30°C across flow and return
Keep flue vent system clean and clear
Inspect for flame detected without demand or burner off with heating block call
Regular test of safety temperature limiter, hot water sensor, and differential sensor
Trusted Resources
Worcester Bosch Support: https://www.worcester-bosch.co.uk
Gas Safe Register: https://www.gassaferegister.co.uk
UK Gov Boiler Efficiency: https://www.gov.uk
Frequently Asked Questions
What does Worcester C4 error mean?
It means the boiler fan or flue system is faulty, or the system can’t detect airflow or combustion readiness.
Can I fix a Worcester C4 boiler fault myself?
Only basic resets or visual flue checks. Fan, sensor, or wiring faults require a Gas Safe engineer.
How long does it take to fix a C4 fault?
With parts on hand, we resolve most C4 errors within 1–2 hours.
Does the C4 error affect the gas supply?
Yes. It prevents gas flow to the burner for safety. Gas valve and gas ratio control valve may be locked out.
Why does my Worcester Bosch boiler keep showing error code C4?
Recurring faults often indicate sensor failures, software mismatches, or pressure irregularities.