After upgrading 2.1.5 to 2.1.6 product images are no longer visible on category pages however they are visible on the product page?
The console log says the image is 404 not found?

This is expected behavior (unfortunately).
This is also a duplicate of https://github.com/magento/magento2/issues/9275 & https://github.com/magento/magento2/issues/9276
Please see this for more info and a possible workaround to be executed before you upgrade (highly experimental): https://github.com/baldwin-agency/magento2-module-magento-216-image-resize-migration-helper
I have run the command:
magento catalog:images:resize
and the images have appeared.
You can close this.
@bondimedical3: please close yourself, I can't do that for you. (I'm not a Magento employee :))
Thank you for your valuable assistance.