The boiler has lost contact with the thermometer that measures the temperature in your home, so it may not adjust heating as accurately, though it will often still heat your house and water.

DIY fix possible

Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 2 minutes.Check the cable from the room temperature sensor or room control unit for loose, damaged, or disconnected wires, and reconnect any loose ends at the sensor end only. Do not open the boiler casing.Restore power, press the reset button on the boiler or controller, and check whether code 57 clears. If it returns, call a Gas Safe registered engineer.