how can i fix my combi boiler alpha 240e?

my betzdearborn combi boiler periodicaly clicks out,when this happens an orange light appears next to a symbol that looks like a thermometer.on pressing the reset button the boiler fires back up again and all is well until the next time ,i'm not sure what the symbol relates to as there is'nt a manual to check.can you help as this is becoming a more frequent problem