If your boiler is alternating between a temperature say 40 and the -¦¦- symbol.
Siphon fill mode. The boiler will run at a low burn for about 15 minutes to fill the condense trap. This happens after the boiler has been turned off, if it hasn’t been used for a day or so, and if the control knob has been turned down and back up. This is perfectly normal – it is a safety feature of the boiler. It will last about 15 minutes, the boiler will then go back to normal. If the boiler does this for more than half an hour, please ring your Heating Engineer or Worcester 0330 123 9559