DC31 cuts out (not the filter)

So I bought a used DC31 on ebay and for a while it was fine but now its started cutting out after a few seconds, ive been through all the you tube videos and ive cleaned it , checked the pins , the battery seems ok as I can get a month of use out of it by releasing and repressing the trigger, there is no blockages.

Im stumped I did read something about a thermocouple but no specific details on what where and how.