I reported a crack on my Fenix 5 HR sensor and the watch is still in warranty and they responded with this :
Hello John,
Thank you for the image of your proof of purchase, but a warranty replacement would not be indicated for this case. The appearance of "cracking" is present on all Garmin devices with wrist heart rate monitors. Being provided a device replacement would result in the same thing eventually being present on the replacement.
Thank you for choosing Garmin,
THIS IS NOT ACCEPTABLE! I DEMAND A REPLACEMENT!!