2001 Chevy Cavalier fan does not come on

Hello, I recently noticed my car over heating. Changed my thermostat, changed my coolant sensor, and the only time my fan comes on is when I put the a/c on. Noticed the coolant is getting pushed back up through the over tube tank, I'm thinking because the fan does not come on. Any help would be very helpfull, dont know what to do. Took it to a shop and they said the thermostat was bad after I just changed it, brought the car back home checked the thermosat in hot water and it was fine. Any help would and I would be very Thankful. Thank you