Just bought a 2002 trailblazer LT. It ran perfect until on the way home we turned the fan switch to defrost and then the car started to run rough and at a stoplight at idle the car stopped running. I thought it was air intake and at my garage after returning home pulled the air breather out and it was pretty well clogged. I left it out and the car seem to run a little better, but it did not solve the problem. Could it be a sensor?

Welcome to AF. Start with cleaning the the throttle body.