Baxi 80e ch not getting warm?

My Baxi 80e boiler is heating my water fine but not my ch. There are no fault lights flashing and the timer is working correctly. The ch temp setting is set at max. The burner light comes on and the temp led's come on upto 80 degrees.Within a matter of seconds the burner light goes out and the led's turn off down to 40 degrees. After 3 minutes the burner light goes back on and the cycle starts over again, therefore no radiators get warm. I have changed the temperature sensor but there has been no change. Can anyone help please????