Rapid blinking CEL, only @ idle?

[Signature620]

This happens regardless of the truck make. Absolutely nothing to be worried about, only a warning that it's about to idle shutdown.

Just for reference while we're at it:
Blinking check engine light after startup - Maintenance fault code (low coolant, water in fuel, etc)
Solid check engine light - Active fault code.
Solid stop engine light - Active serious fault code (Low oil pressure, high coolant temp, accelerator circuit, etc)
Flashing stop engine light - Engine will shut down in 30 seconds (very low oil pressure, very low coolant, very high coolant temp, etc)