The last thing you want when the weather turns colder is for your Worcester Bosch boiler to show a fault code and stop heating your home properly.
These Worcester Bosch boiler fault codes are designed to help a certified Gas Safe engineer quickly identify the problem, especially when the issue isn’t obvious at first glance. While fault codes can give a strong indication of what’s wrong, they don’t always tell the full story – so further inspection may still be required.
Because there are so many possible boiler issues, we’ve put together a list of the most common Worcester Bosch fault codes. This quick guide will help you understand what the codes mean, the possible causes, and what action you may need to take to get your boiler working again quickly and safely.
What Are Worcester Bosch Boiler Fault Codes?
What fault codes mean
Worcester Bosch boilers use fault codes as a built-in diagnostic system. When something isn’t working correctly, the boiler displays a combination of letters and numbers on the control panel. Each code relates to a specific type of fault — for example, low water pressure, a faulty sensor, or ignition problems. These codes help narrow down the issue so it can be fixed more quickly.
Why fault codes appear
Fault codes appear whenever the boiler detects that it can’t operate safely or efficiently. Common reasons include:
- Sensor or component failures
- Low or high water pressure in the system
- Problems with ignition or the gas supply
- Overheating due to circulation issues
- Electrical faults such as wiring or PCB (printed circuit board) errors
When to call a Gas Safe engineer
Some minor issues — such as topping up your boiler’s pressure — may be safe for you to check yourself. However, if the fault code relates to gas, ignition, electrical, or overheating problems, you should always contact a Gas Safe registered engineer. They have the expertise and tools to diagnose and repair faults safely.
Worcester Bosch Boiler Fault Code Table
| Fault Code | Fault Description | Common Causes | Suggested Actions |
| 9A 362 | Incorrect HCM (heat control module) setting | Software/configuration issue | Engineer reset/reprogram required |
| 9U 213 | Faulty HCM detected | Control unit fault | Replace HCM – engineer required |
| C6 215 | Fan speed error | Fan issue or wiring fault | Check/replace fan |
| C7 214 | Fan not running | Blockage, seized fan, or wiring fault | Call engineer to check fan and PCB |
| D1 286 | Flow temperature sensor fault | Sensor wiring issue or failure | Replace sensor |
| E9 224 | Safety temperature limiter activated | Overheating, pump failure, blocked system | Turn off boiler, call engineer |
| EA 227 | No flame detected during start-up | Gas supply, ignition electrode, flame sensor issue | Check gas supply, call engineer |
| EA 229 | Flame loss during operation | Low gas pressure, flue issue, or blocked condensate | Engineer investigation needed |
| F0 238 | Flow sensor short circuit | Wiring fault or failed sensor | Replace sensor |
| F7 238 | Mains supply reversed polarity | Incorrect wiring | Electrician/Gas Safe engineer required |
| F7 241 | No mains supply to boiler | Power cut, tripped fuse, faulty PCB | Check power, call engineer |
| FA 306 | Flame detected when no flame expected | Faulty gas valve or PCB | Dangerous — engineer only |
| H07 218 | Low system water content | Low system pressure | Repressurise system if safe |
| H11 365 | Coding card fault | Incorrect or faulty card fitted | Replace card |
| L9 228 | Flue gas temperature too high | Blocked flue, fan fault, sensor failure | Engineer inspection |
| U6 227 | Boiler lockout | Multiple ignition failures | Reset once, call engineer if persists |
| 101 | High water temperature | Blocked system or pump failure | Allow to cool, call engineer |
| 102 | Low water pressure | Leak or underfilled system | Top up via filling loop |
| 103 | No water circulation | Pump fault, airlock, blockage | Bleed radiators, call engineer |
| 104 | Flow temperature sensor fault | Sensor damaged or disconnected | Replace sensor |
| 107 | Heat exchanger blockage | Scale build-up or debris | Powerflush/engineer clean |
| 110 | Boiler overheating | Pump failure, blockage, or sensor issue | Switch off & call engineer |
| 111 | Flow temperature sensor open/short circuit | Wiring or sensor fault | Replace sensor |
| 117 | System pressure too high | Overfilled, faulty expansion vessel | Release pressure, call engineer |
| 118 | Low system pressure | Leak, underfilled system | Top up to 1–1.5 bar |
| 125 | Circulation fault | Pump not working or blocked | Engineer inspection |
| 128 | Flame loss during operation | Low gas pressure, blocked flue, condensate issue | Engineer required |
| 297 | Low mains voltage | Electrical supply issue | Contact electrician |
| 298 | Low external voltage | Supply problem outside boiler | Call utility company/engineer |
| 305 | Boiler reset required | Lockout condition | Reset once, call engineer if repeats |
Common Worcester Boiler Fault Codes Explained
EA 227 Fault Code – No Flame Detected
What the code means
The Worcester Bosch EA 227 fault code appears when your boiler fails to detect a flame during the ignition sequence. In simple terms, the boiler is trying to start but can’t establish a safe and steady flame.
Possible causes
- Gas supply issues (low pressure or interruption)
- Blocked or frozen condensate pipe
- Faulty ignition electrode or flame sensor
- Problems with the gas valve or PCB (printed circuit board)
- Air trapped in the system
What you can safely check
- Make sure your gas supply is working (other gas appliances in the home should function).
- Check if the condensate pipe outside is frozen in very cold weather. If safe to do so, thaw it gently with warm (not boiling) water.
- Try resetting the boiler once using the reset button.
When to call a gas safe engineer
If the EA 227 code keeps coming back after a reset or if you can’t detect an obvious cause, you’ll need a Gas Safe registered engineer. They can test the ignition system, inspect the gas valve, and check for electrical faults inside the boiler.
C6 215 Fault Code – Fan Speed Error
What this code means
The Worcester Bosch C6 fault code indicates that the boiler fan is either running at the wrong speed or has stopped working altogether. The fan is crucial because it pushes waste gases safely out of your flue. If it’s not working, the boiler will shut down for safety.
Possible causes
- Faulty or seized fan
- Loose or damaged fan wiring
- Blockage in the flue or air intake
- Defective PCB (control board)
What you can safely check
- Make sure the flue isn’t visibly blocked outside (e.g. by leaves, ice, or debris).
- Listen for the fan when the boiler tries to fire — if it’s silent, it may have failed.
When to call a gas safe engineer
This is a fault that almost always requires a professional fix. A Gas Safe engineer can test the fan, wiring, and PCB, and replace parts if needed.
118 Fault Code – System Pressure Too Low
What this code means
The Worcester Bosch 118 fault code means the boiler has detected that the water pressure inside the heating system is too low (usually below 0.5 bar). Without enough pressure, the boiler won’t be able to circulate water properly.
Possible causes
- Leak in the heating system (radiators, pipes, or boiler itself)
- Recently bled radiators without repressurising
- Faulty pressure relief valve
- Air in the system
What you can safely check
- Look at the pressure gauge on the front of your boiler. If it’s below 1 bar, it needs topping up.
- Repressurise the system using the filling loop (check your boiler manual for guidance).
- Inspect for visible leaks around radiators and pipework.
When to call a gas safe engineer
If the boiler keeps overheating, call an engineer. They’ll check the pump, heat exchanger, and sensors, and may recommend a system flush if sludge or limescale is causing the problem.
224 Fault Code – Temperature Sensor Fault
What this code means
The Worcester Bosch 224 fault code points to a problem with one of the boiler’s temperature sensors. These sensors monitor water and component temperatures to keep the boiler running safely. If the sensor can’t provide an accurate reading, the boiler shuts down.
Possible causes
- Loose or damaged wiring to the sensor
- Sensor failure (open or short circuit)
- PCB (printed circuit board) error
- Overheating causing false readings
You suspect a leak
If you notice water around your boiler, pipework, or radiators, don’t ignore it. Even small leaks can cause long-term damage and should be dealt with by a qualified professional.
What you can safely check
- Reset the boiler once to see if the fault clears.
- Make sure your system pressure is correct (1–1.5 bar).
What to call a gas safe engineer
If the code reappears after a reset, the sensor or wiring likely needs replacing. This is an internal electrical component, so a Gas Safe engineer must carry out the repair.
101 Fault Code – High Water Temperature
What this code means
The Worcester Bosch 101 fault code shows that the water temperature inside the boiler is too high. The boiler has safety mechanisms that trigger this code to prevent damage or scalding.
Possible causes
- Faulty or blocked pump preventing circulation
- Air trapped in the heating system
- Scale build-up in the heat exchanger
- Incorrect boiler settings
What you can safely check
- Check the pressure gauge (1–1.5 bar is ideal).
- Bleed radiators to release trapped air.
- Reset the boiler once after it cools down.
When to call a gas safe engineer
If the issue persists, an engineer will need to test the pump, sensors, and heat exchanger. Leaving this fault unresolved could cause overheating or repeated lockouts.
110 Fault Code – Boiler Overheating
What this code means
The Worcester Bosch 110 fault code means the boiler has overheated. This happens when water inside the unit rises above a safe temperature, forcing the system to shut down.
Possible causes
- Pump failure (water not circulating)
- Limescale or sludge in the heat exchanger
- Blocked system pipework
- Faulty temperature sensor
What you can safely check
- Check the system pressure (should be 1–1.5 bar).
- Bleed radiators to remove trapped air.
- Allow the boiler to cool fully, then reset once.
What to call a gas safe engineer
Persistent overheating needs professional attention. A Gas Safe engineer can check the pump, sensors, and system circulation, and may recommend a powerflush to clear blockages.
117 Fault Code – System Pressure Too High
What this code means
The Worcester Bosch 117 fault code means the boiler pressure is too high (usually above 2.5–3 bar). Excessive pressure can stress components and trigger safety lockouts.
Possible causes
- Boiler has been overfilled via the filling loop
- Faulty or waterlogged expansion vessel
- Sticking pressure relief valve
- Blocked system pipework
What you can safely check
- Look at the pressure gauge — if it’s well above 2 bar, you may need to reduce it.
- Bleed a radiator carefully to release excess pressure.
- Make sure the filling loop is fully closed.
When to call a gas safe engineer
If the pressure keeps rising or won’t stabilise, call an engineer. They’ll check the expansion vessel and safety valves to prevent damage to the system.
125 Fault Code – Circulation Fault
What this code means
The Worcester Bosch 125 fault code shows that the boiler has detected a problem with water circulation. This means hot water isn’t moving properly around the system, which can cause the boiler to overheat or shut down.
Possible causes
- Pump failure or seized pump
- Blocked or restricted pipework
- Airlock in the system
- Sludge or scale build-up reducing circulation
What you can safely check
- Look at the system pressure (should be 1–1.5 bar).
- Bleed radiators to release any trapped air.
- Listen for unusual noises from the pump or radiators.
When to call a gas safe engineer
If the circulation fault doesn’t clear after topping up and bleeding radiators, you’ll need a Gas Safe engineer. They can test and, if necessary, replace the pump or carry out a system flush to clear blockages.
128 Fault Code – Flame Loss During Operation
What this code means
The Worcester Bosch 128 fault code appears when the boiler’s flame starts successfully but then goes out during operation. To protect the system, the boiler will lock out until the problem is resolved.
Possible causes
- Low or unstable gas pressure
- Blocked condensate pipe
- Faulty ignition electrode or flame sensor
- Flue or airflow issues
- PCB (control board) fault
What you can safely check
- Check if the gas supply is working (other gas appliances in your home should still function).
- In cold weather, inspect the condensate pipe outside for blockages or freezing — thaw gently with warm water if necessary.
- Reset the boiler once to see if it restarts.
When to call a gas safe engineer
If the fault persists after a reset, call an engineer. They’ll test the gas valve, ignition system, and flame sensor to get the boiler running safely again.
Need Help With a Worcester Bosch Boiler Fault?
If your Worcester Bosch boiler is showing a fault code and you can’t resolve it with a simple reset or pressure top-up, it’s time to call in the experts. Many error codes point to problems with gas supply, ignition, or electrical components – all of which must be handled by a Gas Safe registered engineer.
