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.
Preventing the F73 Error in the Future
- Maintain consistent water pressure in the system
- Ensure regular cleaning of the flue gas path
- Avoid temperature change registered faults through correct boiler maintenance
- Check and document the gas flow pressure and match it to the correct gas family
- Keep a log of basic troubleshooting steps and any previous fault codes
Trust 0800 Homefix for Reliable Boiler Repairs
Our engineers:
- Are Gas Safe certified with extensive experience on Vaillant F73 pressure fault
- Carry genuine Vaillant ecoTEC boiler replacement parts
- Understand all boiler error codes, sensor types, and system-specific signals
- Conduct thorough tests including the mass flow sensor, flame detection, and power supplies
We handle everything from internal pressure inconsistencies to incorrect wiring or sensor configuration ensuring safe and lasting repairs.
FAQs
What does F73 on my Vaillant boiler mean?
It means the boiler can’t detect internal water pressure correctly due to a defective or misconnected pressure sensor.
Is this fault dangerous?
No immediate danger, but your boiler’s performance and safety functions may be compromised until repaired.
Can I fix the F73 fault myself?
You can reset the boiler and check the pressure gauge, but internal sensor work should only be done by a professional.
Does the F73 fault mean I need a new boiler?
Not at all. Most cases are resolved with a new pressure sensor or minor wiring repair.
If you’re facing the Vaillant F73 error, don’t delay. Call 0800 Homefix now for reliable diagnosis, Vaillant boiler repairs, and trusted service from Gas Safe engineers.
Book your same-day repair and keep your heating running smoothly.
More info on Valliant Boiler Error codes from the manufacturer here.