Worcester Greenstar R40 He?

Hi I had my Worcester Greenstar R40 HD just had my system drained today as the plumber was chaining a load of plastic pipe back to copper and when we fired up the boiler it stops getting hot at around 37c and an error come up looking like 2 x t's back to back this error isnt in to manual and i cant find online any where
however if we press the chimney sweep mode the boiler works fine and get up to 69c very quickly, please help as I don't want to run in this mode as i don't know what it does.
any help is appreciated