My heater cuts off 30 sec what do i do

few things to check , if its a dual voltage model check that its set for correct voltage ( if your on a 110v supply and the heaters set to 240v , then the fan will spin too slow and cause the over temp stat to activate )
replace the thermocouple ( a metal tube that sticks into the flame path ( usually has a brass/copper tube running out from it to the over temp stat and down to the gas relay valve , be very gentle whilst replacing this as there is a fibre washer /insulator on the end and it can easily be damaged by over tighting (