Hi there,
you could be correct in your original assumption that you have a fault sensor and this is actually pretty common. But, if it isn't then you can either do as you suggest, replace the heatsink with a 3rd party one. However, you may have to use some plastic nuts & bolts to refit everything as the originals can become damaged when you remove them, if you are not careful and 3rd party heatsinks don't always have the mounting holes in the correct position.
Before spending too much money though, a cheaper option is to carefully remove the heatsink and re-apply some new thermal paste, as this may be all that's needed.