E302 means that the system pressure is too low and you may see that the system pressure gauge indicates that the pressure in the system is below the 1 bar pressure.
To re-pressurise the system you will find a filling loop (metal braided flexible hose) on the underside of the boiler with two tap connections at either end of the filling loop. One small blue & one small black tap handles.
The taps should be opened a quarter of a turn. Open the small black one and then open slowly the small blue one. The water will start to refill the system (water should be heard filling the system). You should see the pressure gauge rise back up between 1 bar and 1.2 bar pressure and then stop filling.
Once the pressure gauge is between 1 bar and 1.2 bar turn the taps back to the off position and the water will stop filling the system. The error code E302 will stop and the boiler will now operate. If the error code E302 is still active, fill up the system a little more but not over 1.5 bar.
(If you do fill the system too much and the pressure gauge is too high, just bleed water from a radiator until the pressure on the gauge goes down to below 1.2 bar).
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