* Faulty Window Regulator: This is the most likely culprit. The regulator is the mechanism that controls the up and down movement of the window. The motor might be fine, but the internal components of the regulator (gears, cables, or a broken plastic part) responsible for lowering the window could be broken or jammed.
* Wiring Issue: A broken or frayed wire in the circuit responsible for lowering the window could interrupt the power flow to the motor in the down direction. This is less common than a regulator problem, but possible.
* Switch Problem: While less likely if the window goes up, the window switch itself could be faulty. It might be sending power to raise the window but not to lower it. This is more likely if the issue affects only one window.
* Motor Problem (Less Likely): Although the motor is usually to blame when the window won't move at all, it could have a problem specifically with the down function. This is less frequent than a regulator problem.
* Fuse: While a blown fuse could prevent both up and down movement, it is possible to have a very unusual partial short circuit that affects the down function but not the up. This is relatively rare.
* Overcurrent Protection: Some more advanced systems incorporate overcurrent protection. If the motor draws excessive current during the lowering function (e.g., due to a bind or obstruction), this could trip a protective circuit.
To diagnose the problem accurately, it's best to:
1. Inspect the window mechanism visually: Look for any obvious obstructions or damage to the regulator.
2. Check the window switch: Try cleaning the switch contacts or testing it with a multimeter.
3. Check the fuse(s): Consult your car's manual to locate and check the relevant fuse.
4. Test the wiring: This is more advanced and might require some electrical knowledge.
5. Consider professional help: If you're not comfortable diagnosing electrical problems, it's best to take your car to a mechanic.
Ignoring the problem could lead to further damage, so addressing it promptly is recommended.
