mywarmhome.co.uk · Heating explained
Boiler fault finding
Don’t guess what’s broken. Start with what the boiler is telling you.
01 · Start with the symptom
What is the boiler actually doing?
A useful diagnosis begins with evidence: what works, what does not, when the problem appears and whether a code is displayed.
Stop
If you smell gas or suspect a gas leak, do not use electrical switches or naked flames. Open doors and windows if safe, leave the area and call the National Gas Emergency Service.
02 · A process of elimination
Six safe observations before you call
Step 01
Power
Is the display on? Check only the normal user controls and fused spur.
Step 02
Controls
Is the programmer on and is the thermostat asking for heat?
Step 03
Pressure
Read the gauge and compare it with the range in your boiler manual.
Step 04
System
Note whether every radiator is affected and whether hot water works.
Step 05
Fault code
Photograph the display and record when the code appeared.
Step 06
Engineer
If the fault repeats, leaks or needs the casing opened, stop.
Fault finding is usually a process of elimination — not a guess at the most expensive part.
● You can observe this
Homeowner checks
Use only controls and procedures covered by the user instructions for your exact boiler.
- Displayed pressure and fault code
- Thermostat, timer and programmer settings
- Whether power is present
- Whether other rooms or hot water are affected
- Visible external leaks or a frozen external condensate pipe
● Engineer check
Do not open the casing
Internal tests and anything affecting combustion or gas safety belong with a suitably qualified Gas Safe registered engineer.
- Gas valve, burner and ignition components
- Fan, flue and combustion checks
- Pump, sensors, valves and heat exchangers
- PCB, internal wiring and electrical tests
- Repeated lockouts, overheating or water near electrics
03 · Quick reference
Find a boiler fault code
Choose the manufacturer and enter the exact code. The result identifies the general fault area — it is not a repair instruction.
Important: codes can differ between boiler ranges and generations. Confirm the meaning in the user manual for the full model number. Never rely on a code alone to order parts or attempt a repair.
Enter a code to see a plain-English explanation.
Resetting
Don’t just reset it.
A reset does not repair a fault. It simply gives the boiler permission to try again.
If a lockout returns, record the code and arrange an engineer visit.
Pressure
Where did the pressure go?
Topping the boiler up is not the repair. If pressure keeps falling, the system needs investigating.
Common symptoms
What your observations may point towards
These are broad fault areas, not a remote diagnosis. The same symptom can have several causes.
No heating
Safe observations
- Is the thermostat calling for heat?
- Is the timer or programmer on?
- Are radiator valves open?
- Does hot water still work?
Possible technical areas
- Control or zone-valve fault
- Circulation problem
- Diverter-valve fault on a combi
- Boiler lockout or sensor fault
No hot water
Safe observations
- Does the heating still work?
- Does the boiler react when a hot tap opens?
- Is the issue at every hot tap?
- For a cylinder, is hot water scheduled?
Possible technical areas
- Diverter valve
- Plate heat exchanger
- Flow or temperature sensor
- Cylinder control or zone valve
Pressure keeps dropping
Safe observations
- Record pressure when cold and when hot
- Look for visible leaks without removing covers
- Note whether radiators were recently bled
- Follow only your model’s user instructions
Possible technical areas
- Leak in the heating circuit
- Expansion-vessel issue
- Pressure-relief valve discharge
- Pressure sensor or gauge fault
Radiators are not heating properly
Safe observations
- Is every radiator affected?
- Cold at the top, bottom or all over?
- Are both valves open?
- Is the boiler reaching temperature?
Possible technical areas
- Trapped air or poor balancing
- Stuck thermostatic radiator valve
- Sludge or restricted circulation
- Pump or control issue
Boiler or system is making noise
Safe observations
- Describe the sound: bang, hum, whistle or gurgle
- Note when it starts and stops
- Check the displayed pressure
- Record a short video for the engineer
Possible technical areas
- Air or poor circulation
- Pump or valve vibration
- Limescale or system debris
- Overheating or combustion issue
04 · Know when to stop
A fault code is information. It is not a repair instruction.
Record the symptoms, pressure, exact code and boiler model. That gives a qualified engineer a far better starting point.