The fan should run constantly, it may well be the fan element has failed\starting to fail. The other modes will use a different element hence why it heating up ok.
To check the fan and the fan element turn the oven on fan mode then set the temp to high, and place your hand at the back of the oven, and check you can feel warm\hot aim comming from the back.
Close the oven leave it for 15 minutes then repeat the process again and check the fan is blocking hot air.
If the fan has stopped this is a fan fault (it should never stop), if the air is warm (not hot) then this would suggest an element fault providing the temp is set to high\max.