Sounds like your Silverado's EGR monitor is having a hard time running and subsequently, becoming ready. There is a way to speed up the EGR monitor running. It involves following the Chevy Silverado Drive Cycle's EGR monitor portion; which we indicate below along with a link to the entire Chevy Silverado Drive Cycle which we recommend you complete at least twice. Be cautious of traffic.
Chevy Silverado EGR Monitor Portion of Drive Cycle
Prerequisite: No trouble codes present, Engine Coolant Temperature (ECT) is above 75 Degrees (F); barometric pressure (BARO) above 72kPa.
1. Idle vehicle 5 minutes (achieve closed loop)
2. Accelerate and drive for 1 min at steady speed above 50 mph
3. Slow vehicle down to 20 mph. Do not use brake clutch, or shift gears.
4. Repeat steps two and three 5 times.
GM and Chevy run their EGR monitor during the deceleration portion of the drive cycle.
Follow the link below for the entire Chevy Silverado Drive Cycle:
Chevy Silverado Drive Cycle Step by Step
