UN 13,402 is sequenced with UN 13,015, even though it lies across the Canon Diablo valley. One way mileage and elevation gain are measured from the summit of UN 13,015. Round-trip mileage and elevation gain assume completion of the sequence as described.
To get to UN 13,402, you can either walk all the way around on the ridge that circles the valley, which will add up to numerous miles, or you can drop back down into Canon Diablo, and then ascend 1,900 feet back up to UN 13,402. Though the extra elevation is not enticing, overall, we figured it would save time, so we picked a line down to the SE and began dropping off the summit, following down a shallow gully that was at first, a scree gully that lower down gave way to more grassy tundra. The descent goes quickly down to about 11,500 ft. We found the partially devoured carcass of an elk here and some fresh bear scat nearby. This made us a little more alert as we continued on through the open forest, heading towards two couloirs coming off of 13,402 from the saddle just SW of the summit. The map shows a small tributary flowing west down one of the two couloirs and then turning abruptly north. We started out following that drainage, but turned into the more northern of the two possible couloirs to begin our ascent. The other may have been less steep, by just a little.
As soon as you exit the trees at 11,600 ft., the terrain gives way almost immediately to a relentless boulder field that stretches for about 600 feet in elevation before the gully narrows and becomes much steeper. For the next 800 feet, you will be scrambling on boulder type talus, with each stacked loosely atop the other, and there is never any secure footing. On the left side of the gully are some spectacular spires of rock to intrigue as you hammer away at this slope. The gully will finally spit you out onto the SW ridge near the saddle and from there, it is a much easier stroll for another 400 feet to the even broader summit. For us, the entire traverse took 2:35. We were getting tired.
Take a well-deserved break on the summit. There was a register in 2012. Using our friends beta, we headed down from the summit following the broad north ridge and making sure to avoid the mistake of heading off into terrain to the right and a small lake lower down. Continue down the ridge, the rock gradually giving way to more tundra and the ridge making a swing more to the north, then NW. By detouring a little to the left (south) side of this broad ridge as you near the trees, you can avoid the trees until almost back to the faint trail in Canon Diablo. In fact, we came back on the trail only a few yards further up canyon from where we had first departed it in the morning.
Once back on the needle-covered trail, hike back down, hopefully able to follow more of the trail than in the morning. This old trail actually led us down, staying closer to the creek until the creek took a turn toward the NE. In this lower, flatter area, we eventually lost the trail, but went ahead and crossed Diablo Creek on some logs, climbed back up an embankment and then walked NE, between the two creeks for a while, before deciding to cross Cochetopa Creek. Where we crossed it was just upstream of the narrow gorge section. We again removed our boots and took a refreshing wade across. Begin the 4.25 mile hike back to the trailhead. This will be a long trudge. Our entire day was almost eleven hours. That included two, half-hour breaks on each summit.
Links to other information, routes & trip reports for this peak that may be helpful.