* Cable Issue: The most common culprit is a frayed, broken, or stretched interior door handle cable. This cable connects the inside handle to the latch mechanism. When it's damaged, it might not fully release the latch.
* Latch Mechanism Problem: The latch mechanism itself could be worn, jammed, or broken. Over time, dirt, debris, or rust can interfere with its smooth operation.
* Rod Linkage: There's a linkage rod that connects the handle mechanism to the latch. This could be bent, broken, or dislodged.
* Door Striker: The striker plate (the metal piece on the door frame that the latch engages with) could be bent or misaligned, preventing the latch from fully disengaging.
* Frozen Mechanism (if cold weather): In colder climates, lubricant in the mechanism could freeze, making it stiff.
Troubleshooting Steps:
1. Lubrication: Try spraying some WD-40 or a silicone-based lubricant into the latch mechanism (be cautious not to spray directly onto electrical components). Work the handle several times to distribute the lubricant. This might help if it's a minor stiffness issue.
2. Inspect the Cable (if accessible): If you can access the cable mechanism (often involves removing the door panel), inspect it for fraying, breakage, or stretching. This often requires some mechanical aptitude.
3. Check the Linkage: Similarly, accessing the linkage rods might require removing the door panel. Check for bending, breakage, or dislodgement.
4. Examine the Latch Mechanism: If you can access the latch mechanism (after removing the door panel), check for any visible obstructions, rust, or damage.
5. Check the Striker Plate: Inspect the striker plate on the door frame. See if it's bent or misaligned. If bent, you may be able to carefully adjust it with pliers.
6. Try the Exterior Handle: If the door opens easily from the outside, it points toward an interior handle or cable issue.
If you're not comfortable working on car doors yourself: Take your GMC Sierra to a mechanic or a body shop. They have the experience and tools to diagnose and repair the problem accurately. Replacing the cable or latch mechanism is usually a straightforward repair for a professional.
Important Note: Removing the door panel requires some mechanical skill. If you're not familiar with this procedure, consult a repair manual specific to your 1998 GMC Sierra to avoid damage. You'll likely need various tools like screwdrivers, panel removal tools, etc.
