Seat belt shifter interlock, Gen 1, non-customizable

Years 2023–25, 2027 Models 2 Factory citations 5

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Also described as Buckle to drive

Also documented for Chevrolet

Documented applications

Sierra 1500 — Next-Generation (2027)

2027 Factory-verified

Pro, Elevation 3SB, AT4, AT4X, Denali, Denali Ultimate: O; Elevation 3VL: not offered; prevents shifting out of Park until the driver seat belt is fastened; times out after 20 seconds

GM factory document — 2027 gmc next generation sierra 1500 retail and fleet fleet order guide, p.32.
GM factory document — 2027 gmc next generation sierra 1500 retail and fleet fleet order guide rpo codes, p.1.

Sierra — T1XX (2019–2026)

2023–2025 Factory-verified

the Silverado T1XX doc reads "Buckle to Drive"

GM factory document — 2023 gmc 23 sierra 1500 electrical body builder sm en us 2022oct12, p.14.
GM factory document — 2024 gmc 24 sierra 1500 sm electrical body builder esid 23dec12, p.16.
GM factory document — 2025 gmc my25 sierra 1500 electrical bbm, p.14.

This page aggregates every documented appearance of T4Z in the GMC reference. Confidence and citations are shown per application — the same code can be better documented in one year than another.