* Faulty Switch: The most common cause is a malfunctioning rear defogger switch itself. The switch may be internally shorted, causing it to remain "on" even when turned off. This is often a simple, inexpensive fix requiring only a switch replacement.
* Wiring Issue: A short circuit or break in the wiring harness leading to or from the switch can also cause the system to stay energized. This could be due to damaged insulation, corrosion, or a wire rubbing against a metal component. Finding the short requires careful inspection of the wiring.
* Relay Problem: The rear defogger often uses a relay to control the high current needed for the grid. A stuck relay (either permanently closed or failing to release) will keep the power flowing to the defogger.
* Timer Malfunction (if equipped): Some systems have a timer to automatically shut off the defogger after a certain time. If this timer is faulty, it might not shut off the system. This is less likely, but possible.
* Fuse Problem (less likely): While a blown fuse would *prevent* the defogger from working, a partially shorted fuse could potentially cause it to stay on. This is less likely but worth checking (but only with the ignition OFF).
* Body Control Module (BCM) Issue (unlikely, but possible): In some vehicles, the BCM controls various electrical functions, including the rear defogger. A faulty BCM could theoretically cause this issue, but it's less probable than the other causes listed.
Troubleshooting Steps:
1. Check the fuse: Locate the rear defogger fuse in the fuse box (usually under the hood or in the interior fuse panel) and visually inspect it. A blown fuse will be visibly damaged. However, as mentioned above, a partially shorted fuse is less likely but possible. Remember to turn off the ignition before checking fuses.
2. Test the switch: With the ignition off, try wiggling the rear defogger switch. If you hear clicking or see sparks, it’s likely the switch itself.
3. Inspect the wiring: Carefully check the wiring harness leading to and from the rear defogger switch and the defogger grid itself for any visible damage, corrosion, or broken wires.
4. Locate and test the relay (if applicable): Consult your vehicle's repair manual to identify the relay. You might be able to test it with a multimeter.
If you're not comfortable with electrical work, it's best to take your Buick Century to a qualified mechanic for diagnosis and repair. Attempting to fix electrical issues without proper knowledge can lead to further damage or even injury.
