* Resistor Pack: This is the most common cause. The resistor pack controls the blower motor speed. If it fails, it can cause the motor to run constantly, even on the highest setting. A shorted resistor is often the culprit.
* Blower Motor Control Module (BCM): The BCM is the "brains" of the HVAC system. If it malfunctions, it might fail to send the correct signals to shut the blower off. This is less common than a resistor issue, but still a possibility.
* Wiring Issues: A short circuit in the wiring harness leading to the blower motor or the resistor pack can also cause the motor to stay on. This could be from damaged wiring, corrosion, or a pinched wire.
* Ignition Switch: While less likely, a faulty ignition switch could be failing to properly cut power to the blower motor circuit when the key is removed. This is less probable than the other issues.
* Aftermarket Radio Installation: If a stereo system was recently installed, it's possible that the wiring was incorrectly connected, leading to a short or improper power supply to the blower motor.
Troubleshooting Steps (in order of likelihood and ease):
1. Check the Resistor Pack: Locate the resistor pack (often behind the glove box or under the dash). Look for any signs of burning, melting, or damage. A visual inspection is the first step. If it's visibly damaged, it needs replacement.
2. Check the Blower Motor Switch: Try cycling the blower switch through its positions several times. It could be stuck mechanically.
3. Test the Blower Motor Fuse: Check the fuse for the blower motor in the fuse box. If it's blown, replace it, but if it blows immediately again, there's a short circuit somewhere in the system.
4. Professional Diagnosis: If the above steps don't reveal the problem, it's time to consult a qualified mechanic. They have the tools and expertise to diagnose more complex electrical issues, such as problems with the BCM or wiring harness. A scan tool might be needed to check for diagnostic trouble codes (DTCs).
Safety Note: Working with car electrical systems can be dangerous. If you're not comfortable working with car electronics, it's best to leave the diagnosis and repair to a professional. A constantly running blower motor can drain your battery quickly.
