Error 204

Safety precautions:

1) If you smell gas, evacuate the property and call the gas emergency number immediately. Do not attempt to operate the boiler.

2) Before touching any boiler controls or external pipework, switch the boiler electrical supply off at the isolator. Only visual/temperature checks and control adjustments should be attempted by a homeowner.

3) Be aware surfaces and pipes can be hot; allow cooling time or use a glove when feeling pipes.

Initial checks a homeowner can do:

1) Check the display and note the exact fault code(s) and any other messages. If possible note the time and frequency of the fault.

2) Wait a few minutes — code 204 can be transient. If it clears by itself and heating resumes, monitor for recurrence.

3) Press the boiler reset button once (follow manufacturer guidance). If the code returns immediately or the boiler locks out repeatedly do not keep resetting; record what happens and call an engineer.

4) Check room thermostat and programmer: ensure the heating is actually set to ON and the target temperature is above the current room temperature. If the thermostat is satisfied it will prevent the boiler from firing.

5) Check the boiler mode: confirm the unit is in the correct mode (CH/DHW/auto) for the demand you expect.

6) Check CH flow temperature settings: if your controller allows, look at the boiler flow temperature set point or the BC10 dial. If the set point is lower than the actual measured flow temperature the boiler will wait until the water cools.

7) Check system pressure on the gauge (normally around 1.0–1.5 bar cold). Very low pressure can affect operation, but code 204 is about temperature differences rather than low pressure.

8) Check radiators and pipe temperatures: when heat is called for, the flow pipe should be noticeably hotter than the return pipe. If both are similar in temperature it suggests poor circulation.

9) Bleed radiators to remove trapped air which can inhibit circulation. Ensure TRVs (thermostatic radiator valves) are open on radiators you expect to heat.

Specific diagnostic and simple fix steps:

1) Access the boiler information menu (press the spanner/return or the info button as per the manual) and read the cause/diagnostic code that appears with the fault. Record this code — it helps an engineer pinpoint the issue.

2) If the information menu confirms the flow temperature reading is higher than the set point, check whether this is expected (recent boiler run) or not. If expected, allow the system to cool and the boiler should start again when temperature drops or when another heat demand arises.

3) If the boiler refuses to fire despite a clear heat demand: check the pump is running. With the electrical supply on and heat demand active, carefully feel the pump casing for vibration or warmth (do not attempt to disassemble). A silent or cold pump during a call for heat suggests circulation failure — power off and call an engineer.

4) If radiators remain cold while the pump sounds to be running, try bleeding radiators fully then re-check. If the problem persists, circulation blockage, valve or diverter problems are possible causes.

5) If the 204 condition is persistent and you also see other related sensor codes (for example codes referencing flow sensor, return sensor, or safety sensors), suspect a faulty temperature sensor or wiring fault. Do not attempt to replace sensors or open the boiler; this must be done by a Gas Safe engineer.

6) If you recently had power cuts or maintenance, occasionally the control board or burner module needs a service reset which an engineer should verify.

When to call a professional (and what to tell them):

1) Call a Gas Safe registered engineer if the 204 code does not clear after basic troubleshooting and reset, if you have no heating or hot water, if you hear unusual pump noises, or if you see additional fault codes alongside 204.

2) Tell the engineer the exact boiler model (Worcester Bosch GB162), the displayed fault code(s), any cause code visible in the information menu, what you have already tried (reset, bled radiators, checked thermostat, checked pressure), and whether the condition is continuous or intermittent.

3) Do not attempt to open the boiler casing, replace sensors, work on gas valves, or perform electrical repairs yourself. Those tasks require a qualified, Gas Safe engineer.

Final notes:

1) In many cases a 204 is temporary and linked to temperature measurements and system demand; if it clears and heating works again you can monitor for recurrence.

2) If the fault repeats, an engineer will diagnose sensors, wiring, pump operation, diverter/zone valves, and the control PCB and carry out safe repairs. Record times and patterns of the fault to help the engineer identify intermittent issues.