Error 305

Safety precautions:

1) If you smell gas, evacuate the property immediately, do not operate electrical switches or the boiler, close the gas supply at the meter if it is safe to do so and call the gas emergency number right away. 2) If there is a leak, water pooling, or electrical burning smell, isolate the electricity to the boiler at the fused spur or consumer unit and call a professional. 3) Do not open the boiler casing, work on gas pipework, or attempt internal electrical repairs yourself—these must be done by a Gas Safe engineer.

Initial checks a homeowner can do:

1) Check whether hot water is actually being used anywhere (tap, shower, dishwasher). If a tap is open, close it and wait a few minutes to see if the code clears. 2) Check the programmer/timer and any external controls (room thermostat, cylinder thermostat if fitted) to confirm no scheduled DHW demand is active. 3) Check the boiler display and note any other fault codes or a flashing warning triangle. 4) Check the system pressure on the boiler pressure gauge—if very low (below about 0.8–1.0 bar) repressurise per the boiler manual, then observe behaviour. 5) If the condensate pipe could be frozen (cold weather) or blocked, inspect the external condensate run for ice or blockage and defrost/clear if safe to do so.

Simple reset and observation steps:

1) If there is no smell of gas or other safety concern, try a basic reset: follow the GB162 reset method (press and hold the reset button for 3 seconds or power cycle the boiler) and watch whether the 305 cause clears and the boiler returns to normal. Only reset once; repeated resets do not fix underlying faults. 2) Record when the code appears (e.g. when using a hot tap or right after) and whether it clears after a short period. 3) Use the spanner/return/info button to read the cause code history on the boiler display and confirm 305 is the current cause.

Further diagnostic checks (for homeowner observation, not for internal repair):

1) Test response: open a hot tap briefly—does the boiler fire and supply DHW? Close the tap—does the boiler switch out of DHW mode and clear the code? If the boiler fires for DHW but then stays locked waiting, that suggests the boiler still senses DHW demand or a sensor/valve is not reporting correctly. 2) If you have mixed heating and hot water behaviour (for example hot water only, no heating, or vice versa), note these symptoms—they help the engineer diagnose diverter valve or sensor faults.

When to call a Gas Safe engineer:

1) If error 305 persists indefinitely or recurs frequently after simple resets and checks. 2) If you have no hot water when you expect it, or if other fault codes appear with the 305 cause. 3) If you hear unusual noises, notice leaks, or see pressure dropping. 4) If you are uncomfortable performing the checks above.

What the engineer will check and likely do:

A Gas Safe engineer will read the boiler’s fault and cause history, test DHW and CH sensors (NTC thermistors), inspect and test the diverter valve and its actuator, check wiring and the heat control module/PCB, and verify correct pump operation and flow sensing. If a sensor or actuator is faulty it will be replaced and the engineer will clear the fault and test multiple DHW cycles to confirm the problem is fixed.

Final note: 305 is often a temporary wait-state but persistent displays indicate a component or control issue. Do the safe checks listed above and call a Gas Safe registered engineer if the code does not clear, you lack hot water, or you find any safety concerns. Do not attempt gas or internal electrical repairs yourself.