Error code
Worcester Bosch Boiler Fault Code 104
System pressure or circulation condition is abnormal The most likely cause is circulation pump not moving water correctly. Check first: check external controls, thermostat demand, and the visible pressure reading where your model provides one.
Quick answer
104: System pressure or circulation condition is abnormal
Meaning
System pressure or circulation condition is abnormal
First safe check
Read the display and note whether the code returns immediately or only during heat demand.
Stop if
Do not remove the casing, adjust gas components, work on combustion parts, or bypass safety devices.
Who to call
Contact a qualified heating or gas professional if 104 returns after one permitted reset, the boiler repeatedly locks out, pressure behaves abnormally, ignition is affected, overheating occurs, or any internal diagnosis is needed.
Medium severity: check safely and stop if it returns. Keep checks outside the casing and use only a manufacturer-approved reset where the manual allows it.
Fix time
A few minutes for external checks
Difficulty
Professional-only beyond display and controls
This is mainly a fault-area lookup for safe escalation, not a DIY repair path.
Expected outcome: A temporary control condition may clear once. Repeated 104 needs qualified diagnosis.
What does this error code mean?
Worcester Bosch boiler fault code 104 usually means system pressure or circulation condition is abnormal. Boiler codes can be model-dependent, so this page is intended to explain the likely fault area and the safe boundary for a user, not to walk through internal repair.
Stay outside the boiler casing. User checks should be limited to the display, thermostat or programmer settings, visible pressure reading where relevant, and one manufacturer-approved reset only if the manual allows it.
This code sits in the circulation fault family. Internal testing should be handled by a qualified heating or gas professional.
Symptoms users may notice
- The boiler may lock out and stop heating or hot water demand.
- The code may return during ignition, heating, or sensor checks depending on the fault family.
- External controls may appear normal while the boiler remains locked out.
Why is this happening?
- Circulation pump not moving water correctly
- Blocked filter, sump, or spray path
- Flow sensor not reading expected movement
- Control or pump wiring fault
What this usually points to
This code sits in the circulation fault family. Internal testing should be handled by a qualified heating or gas professional.
How to fix this safely
- Read the display and note whether the code returns immediately or only during heat demand.
- Check external controls, thermostat demand, and the visible pressure reading where your model provides one.
- Try one manufacturer-approved reset only if the manual allows it and there is no smell of gas, overheating, leak, or electrical concern.
- Stop if the fault returns, the boiler repeatedly locks out, or the next step would involve casing access.
Can I fix this myself?
NoBoiler checks should stay limited to external controls, visible readings, and one permitted reset. Internal diagnosis is professional-only.
Safety boundary: Do not remove the casing, adjust gas components, work on combustion parts, or bypass safety devices.
What not to do
- Do not remove the boiler casing.
- Do not adjust gas valves, combustion parts, flues, fans, or internal wiring.
- Do not keep resetting a boiler that repeatedly locks out.
Common misdiagnoses
- 104 can be treated as a simple reset issue, but repeated boiler lockouts usually need professional diagnosis.
Reset steps
- Check the manual for the permitted reset method on the exact model.
- Reset once only if there is no smell of gas, leak, overheating, or electrical concern.
- If 104 returns, leave the boiler off and contact a qualified heating or gas professional.
Intermittent vs persistent faults
A one-off 104 may clear after a permitted reset, but repeated returns should be treated as persistent and professional-only.
Country safety notes
UK
Gas boiler work should be carried out by a Gas Safe registered engineer.
When to call a heating professional
Contact a qualified heating or gas professional if 104 returns after one permitted reset, the boiler repeatedly locks out, pressure behaves abnormally, ignition is affected, overheating occurs, or any internal diagnosis is needed.
Safety warning
Do not remove the boiler casing or attempt internal repairs. Gas, combustion, flue, fan, sealed-case, and internal electrical work should be handled by a qualified professional.
Useful hub links
Related circulation fault codes
Worcester Bosch Boiler Fault Code 101
Boiler has detected an overheat condition
Worcester Bosch Boiler Fault Code 110
Model-dependent overheat or high-limit-style lockout
Worcester Bosch Boiler Fault Code 200
Heating demand or controls signal issue
Worcester Bosch Boiler Fault Code 102
Flow or return temperature sensor fault
Relevant guide
Frequently asked questions
What does Worcester Bosch boiler code 104 mean?+
104 usually means system pressure or circulation condition is abnormal. The exact cause can vary by model and installation.
Can I reset a boiler with 104?+
Only try one manufacturer-approved reset if the manual allows it and there is no immediate safety concern. Do not keep resetting repeated lockouts.
Can I fix 104 myself?+
No. Stay outside the casing and leave gas, combustion, sensor, PCB, fan, flue, and internal electrical checks to a qualified professional.
Who should I call for 104?+
Use a qualified heating or gas professional. In the UK, gas work should be handled by a Gas Safe registered engineer.
Model variation note
Worcester Bosch boiler code meanings can vary by model range. Check the exact model manual before assuming one failed component.