Q: a leak comes from under dash on passenger side when ac is on but works ok other on 2007 Chevrolet Cobalt

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
then that.
(1) Answer
(1) Comments
If it smells or looks like coolant, presure test the cooling system and see if you can see it leaking inside the vehicle while under pressure. If so, the heater core is leaking. If it isn't coolant, then the evaporator drain is plugged up and the condensation is collecting and backing up and coming out of the box inside the car. The evaporator drain plugging is pretty common on those, so I wold say pull off the rubber drain hose under the hood and clean out the drain anyway whether or not you find the heater core leaking. Only takes a few minutes.