First ever mechanical diversion after 1M+ miles on United.

Was on 1846 last night EWL-SLC. We were next in line for take-off when we pulled into the "line up & wait" position, but then taxied to the first turn-off and sat for 20 minutes while the crew "did some paperwork for a mechanical issue."

Was listening to LiveATC but never heard the exact issue. Eventually they said good to go and we uneventfully took off.

~30 minutes in the captain announced that we were diverting to IAD for a mechanical issue. "It's not really something we can continue with across country." I still never heard the exact issue over ATC.

Landed ~20 minutes later uneventfully but there was a full complement of emergency vehicles (at least half a dozen) with full red lights cranking just off the runway. As soon as we slowed to a stop they circled the plane.

Taxied to the gate. Deplaned and was informed we'd be switching to a new aircraft. Boarded a just landed inbound flight about 2 hours later with a brand-new crew.

Everything was handled smoothly & competently, and having a full backup crew ready to go prob saved us a night.

https://www.flightradar24.com/data/f...a1846#32a2654e