NA4 Factory-verified
Also described as Emission system, GVW over 8,500 lbEmission system, GVW above 8,500 lbsEmissions, GVW over 8,500 lbEmission system, GVW above 8,500 lbEmission system, GVW greater than 8,500 lb
Also documented for CadillacChevrolet
Documented applications
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2005–2007: heavy-duty models only (GM’s HD list)
This page aggregates every documented appearance of NA4 in the GMC reference. Confidence and citations are shown per application — the same code can be better documented in one year than another.