1. Actuator Problems: The most likely culprit is a faulty door lock actuator in one or both front doors. These are the small electric motors inside the door that move the locking mechanism. They can wear out, get jammed with debris, or suffer from internal failures. The fact they *move* suggests they're getting power, but not enough to fully complete the locking/unlocking cycle.
2. Linkage Problems: The actuator is connected to the locking mechanism via a series of rods and linkages. These can become bent, broken, or disconnected, preventing the lock from fully engaging or disengaging.
3. Vacuum System Issues (Less Likely on '99): Older Suburbans sometimes used vacuum systems for door locks. While less likely on a '99 (they likely use electric actuators), a leak in the vacuum lines or a malfunctioning vacuum pump could still cause inconsistent locking.
4. Wiring Issues: A broken or loose wire in the door harness can interrupt the power to the actuators, leading to inconsistent operation. This is often exacerbated by the constant flexing of the wiring as the door opens and closes.
5. Central Locking Module (Rare): While less probable, a problem with the central locking module itself (the electronic control unit for the locks) could also be at fault.
How to Troubleshoot and Fix:
1. Check the Obvious: Start by carefully inspecting the door lock mechanism itself. Look for any obvious obstructions or damage to the linkage. Try manually locking and unlocking the doors with the key to see if the mechanism moves freely.
2. Test the Actuators (Intermediate): You'll need a multimeter or a 12V test light. With the door panel removed (search online for a guide specific to your Suburban year), you can test if the actuators are receiving power when the unlock button is pressed. If you have power but the actuator doesn't move, it's likely faulty and needs replacement.
3. Inspect Wiring (Advanced): If the actuators are receiving power, examine the wiring harness for any breaks, frayed wires, or corrosion. Pay close attention to where the wiring enters the door.
4. Lubrication (Easy Try): Before replacing parts, try lubricating the lock mechanism with a silicone-based lubricant. This might free up a sticky linkage.
5. Replace Actuators (Most Likely Solution): If you've ruled out other issues, replacing the faulty actuator(s) is the most likely solution. These are relatively inexpensive and available from auto parts stores. You'll need to remove the door panel to access and replace them. There are many online tutorials for this.
6. Professional Help: If you're not comfortable working on your car's electrical system or are unable to identify the problem, take it to a qualified mechanic.
Important Note: Always disconnect the negative battery terminal before working on any electrical components in your vehicle. This will prevent accidental shorts or damage to your car's electrical system.
