Why does the blower fan in my 1996 C1500 truck work all speeds but high?

The most likely reason your 1996 C1500's blower fan works on all speeds except high is a problem with the blower motor resistor. This resistor is a crucial component that regulates the voltage going to the blower motor, allowing for different speeds. When it fails, the high speed circuit often gets cut off, leaving only the lower speeds operational (which bypass parts of the resistor).

Other, less likely possibilities include:

* Blower motor itself: While less probable if lower speeds work, a failing blower motor *could* have trouble handling the higher voltage and amperage demanded by the high speed setting. This is less likely because you'd likely see some performance issues even at lower speeds if the motor were failing.

* Wiring issues: A break or short in the wiring specifically for the high speed circuit is possible, though less likely than a resistor problem.

* Fuse or relay: Though less common, a blown fuse or a failing relay dedicated to the high-speed circuit could also cause this. Check your fuse box and relay panel.

* Switch: A faulty blower motor switch could be failing to properly engage the high speed circuit.

Troubleshooting steps:

1. Check the blower motor resistor: This is the most likely culprit. Locate it (consult your owner's manual or online resources for the location, often near the blower motor itself or under the dash). Inspect it visually for any signs of burning, melting, or damage. If it looks suspect, replace it. This is usually a relatively inexpensive and easy repair.

2. Check fuses and relays: Check the fuses and relays associated with the blower motor and its various speeds (again, your owner's manual is invaluable here).

3. Test the blower motor: If the resistor and fuses/relays check out, you might need to test the blower motor itself with a multimeter to ensure it's receiving power and functioning correctly at all speeds. This is more involved and may require some electrical knowledge.

4. Inspect the wiring: If all else fails, visually inspect the wiring harness leading to the blower motor for any signs of damage, shorts, or loose connections. This can be a more time-consuming task.

Replacing the blower motor resistor is generally the easiest and most effective first step in fixing this common problem. If that doesn't solve it, move on to the other troubleshooting steps. Remember to always disconnect the battery negative terminal before working on any electrical components.