Primary color, exterior, Sonoma Red Metallic

Years 2009, 2013–14 Models 2 Factory citations 3

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Also described as Primary color, exterior, Sonoma Jewel Metallic

Also documented for Chevrolet

Documented applications

Sierra — GMT900 (2007–2013)

2009 Factory-verified

The list marks it (07).

GM factory document — 2009 gmc sierra 2wd rpo code list, p.1.
2013 Single-source

GM Authority lists 66U as Sonoma Red Metallic for 2013; the 2009 list reads Sonoma Jewel Metallic for the same code. GM Authority 2013 Sierra RPO Central (https://gmauthority.com/blog/2012/04/2013-gmc-sierra-1500-and-sierra-hybrid-rpo-central/)

Sierra HD — GMT900 (2007–2014)

2014 Factory-verified

as the Sierra GMT900 doc's 2013 row

GM factory document — 2014 gmc sierra 2500hd fleet order guide wayback, p.1.
GM factory document — 2014 gmc sierra 3500hd fleet order guide wayback, p.1.

This page aggregates every documented appearance of 66U in the GMC reference. Confidence and citations are shown per application — the same code can be better documented in one year than another.