2016 silverado gas no start

[dmosley73]

i have a 2016 silverado 2500 HD 6.0 gas engine with 27k miles. i have never had an issue with this truck, i bought it with new with 16 miles. took a trip yesterday and put around 500 miles on the vehicle. when i came out today to go some where, the truck wouldn't start. all the lights on the dash come on, when you turn the key, you can hear the relay under the dash, as well as feel the relay in the fuse block engaging. there is no click at the starter. battery has 12.8 volts and doesn't not draw down when you turn the key. snap on scan tool pulled codes p685 p686. both related to pcm power relay. is there a PCM power relay? where is it located? can i test it? i'm a total noob here but i've been doing my own wrenching for 30 years and any knowledge or assistance would be great! thanks