Answer: For most 2017 GMC Terrain models, the tail light uses a dual-filament 3157-series bulb (3157 or 3157A) for tail/brake/turn functions, while the separate reverse light uses a 921 bulb. If your trim has LED tail lights, there may be no replaceable bulb.
Understanding the tail light bulb options
The rear tail light assembly on the 2017 Terrain is a combined unit that typically uses two different bulbs: one for the tail/brake/turn function and another for the backup light. The most common configuration is a 3157-series bulb for the tail/stop/turn and a 921 bulb for the reverse lamp. Some markets or trims may differ, and certain vehicles with LED tail lights do not have replaceable bulbs in the rear lamps.
- The primary rear bulb for tail, brake, and turn: usually 3157 or 3157A (dual-filament). The exact variant (3157 vs 3157A) depends on whether your turn signal is coded as a separate amber element or integrated with the tail lens.
- The backup/reverse light: commonly a 921 bulb (also known as T15 wedge in some regions).
- LED tail lights alternative: on some 2017 Terrain configurations, the tail lamps are LED and do not use replaceable bulbs.
To ensure you buy the correct bulb, check the bulb current in your vehicle, consult the owner’s manual, or inspect the bulb socket in the tail light housing for the exact part number.
How to verify and replace the tail light bulb
Follow these steps to confirm the correct bulb type and perform a replacement if needed.
- Park safely, open the tailgate, and locate the rear tail light housing.
- Remove the lens cover or access panel to reach the bulb socket (you may need to twist or unclip the assembly, depending on your trim).
- Detach the bulb socket and gently pull out the bulb to read the markings on the base (look for 3157, 3157A, or 921).
- Purchase the correct replacement bulb from a reputable brand (e.g., Sylvania, Philips) matching the markings you found.
- Install the new bulb by aligning it with the socket and pushing/twisting it as required, then reassemble the housing and test the tail light, brake, and turn functions.
- If the new bulb does not light, check the related fuse, the wiring, and the other tail light assembly to rule out a broader electrical issue.
If you’re unsure about the exact bulb type for your specific vehicle, consult GM’s parts diagrams for the 2017 Terrain by VIN, or call a GM dealership parts desk with your vehicle’s trim level and region. They can confirm whether your tail lights use 3157/921 bulbs or an LED unit.
Considerations by model and lighting technology
Incandescent vs. LED tail lights
Most older or lower-trim Terrain models use incandescent bulbs in the rear lamps, which means you’ll replace bulbs periodically. Some higher trims or regional variants may come with LED tail lights, which typically have a non-replaceable light source inside the housing. If you have LED tail lights, you’ll need to replace the entire assembly or module when a component fails, rather than swapping individual bulbs.
When replacing bulbs in any case, always handle bulbs with a clean cloth or gloves to avoid oil transfer, which can shorten bulb life. If you notice moisture, condensation, or unusual behavior in one tail light, have the entire unit inspected by a professional to prevent water damage or wiring faults.
Summary
For a 2017 GMC Terrain, the tail light is most commonly a dual-filament 3157-series bulb (3157 or 3157A) for tail/brake/turn, with a 921 bulb for the reverse light. Some trims may feature LED tail lights, which do not use replaceable bulbs. To be certain, check the current bulb markings, consult the owner’s manual, or verify with a GM parts advisor. Proper replacement ensures the tail lights function correctly and stay compliant with road-use requirements.