Why does the blower motor on 2002 trailblazer work all speeds except lowest?

A 2002 Trailblazer blower motor that works on all speeds except the lowest typically points to a problem with the resistor pack. The resistor pack regulates the voltage going to the motor, allowing for different speeds. When the lowest speed is out, it usually means the resistor in that part of the circuit has failed. The higher speeds bypass that particular resistor, hence why they still work.

Other less likely possibilities include:

* Wiring issue: A break or short in the wiring specifically for the lowest speed. This is less common than a faulty resistor pack.

* Blower motor switch: While less probable if all other speeds work, a failing switch could have a problem with the lowest speed contact.

* Faulty blower motor: Although less likely if the other speeds work, a partially failed motor could still exhibit this symptom. It's usually more likely to completely fail rather than only affect one speed.

In short: Start by inspecting and/or replacing the blower motor resistor pack. This is the most common and easiest fix. If that doesn't solve the issue, then further diagnostics on the wiring and switch may be needed.