A/C overflow

My air conditioner is using the secondary overflow pipe. Why is the primary not being used? What could the problem be?

Drain line.

Your primary line is probably clogged up with debris. If you want to try it, you can take off one of the side panels on the unit, and see what’s blocking the drain line. Make sure you turn off the unit first. You can use a shop vac to blow out the line (from the outside), but I would first try and blow the clog up to the unit, so you don’t lodge the debris into the line, by sucking it at first.

Mike