The boiler's room thermostat sensor appears to be sending a faulty signal, so the boiler may not control the room temperature as accurately as it should, but it is usually not dangerous and can often be fixed simply.

DIY fix possible

Turn off the boiler at its on/off switch and wait 2 minutes, then turn it back on to clear the fault code.If you have a wireless or wired room thermostat (Vitotrol), replace its batteries and check that the cable connections at the thermostat are secure and not damaged.Check the cable from the room sensor for visible damage, such as cuts, pinches or exposed wires, and check that it is not running against a metal surface or in a wet area. If it is damaged, leave the repair to an engineer.If the code returns, do not open the boiler casing. Contact a Gas Safe registered engineer to test and replace the room temperature sensor.