Overheating at red lights on a 2011 4.8 - fans not kicking on?

Overheating at idle is the classic airflow problem, and a fan that isn't switching on is the first thing to check. At a stop, the fan does all the work.

• Confirm the fan actually runs when the engine is hot (and at what temperature).
• Check whatever triggers it — the relay, fuse and temperature trigger.
• Make sure the shroud is in place and covers the core; without it, the fan only pulls air through the section behind its blades.

We don't have a 2011 Silverado wiring source, so use the factory diagram to trace that truck's fan circuit.