Sure it isn't an mid 2012/early 2013 model? These have common GPU issues (which Apple is repairing free of charge until the end of the year), haven't heard of this issue on the late 2013 model before, they are generally quite reliable.
I would check the thermal paste has been applied properly first, if it was refurbished there is a chance it was liquid damaged previously and ultrasonic cleaned in the past (the heatsink would have been removed to do this).
The heatsink itself wouldn't be the issue, unless it is physically snapped or there is a lot of dust between the fan and the heatsink.
Check the model first, take to Apple if it is an affected model https://www.apple.com/uk/support/macbookpro-videoissues/