Why do I get a burn mark from my Apple Watch 2’s sensor?

Hi

If you experience redness, swelling, itchiness or any other irritation or discomfort on your skin around or beneath your watch or band, Apple recommends that you remove your Apple Watch and consult a doctor / physician before wearing it again.

Skin reactions can be a result of various causes including:

• Allergies (eg to nickel).
• Extended exposure to irritants (eg soap or sweat).
• Environmental factors.
• Wearing the band too tightly or too loosely.

More information, including tips from Apple:

• Cleaning and wearing Apple Watch - Apple Support

  Includes "Tips to avoid skin irritation, or if you have allergies or skin sensitivities".

• Wearing your Apple Watch - Apple Support
• See under "Skin Sensitivities":

  Important safety information

If the burn is from excessive heat and you are following Apple's handling instructions here:

• Keep Apple Watch within acceptable operating temperatures - Apple Support
• Important safety information
• Important handling information

then contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:

• Contact - Official Apple Support
• Apple Support (@AppleSupport) | Twitter
• Genius Bar Reservation and Apple Support Options - Apple
• https://locate.apple.com

If you believe this to be a burn, then you need to see a physician, more probably, a dermatologist. It may be that you are allergic to one of the components of the Apple Watch. Wearing your Apple Watch - Apple Support

There is nothing about the watch that could cause a burn, more likely a skin irritation as I mentioned.