I maybe some of help, got Nissan Climate controls credentials and e-609 cert to handle r12 and r134a.
The most common problem would be low refrigerant, but if its leaking, you will need to get a luminous additive and run it under UV light to scan and find where if a leak is present. If no leak is found and a/c is charged (green), then I would check next the thermal expansion valve (TXV) located inside the H block along the firewall side of your engine bay. That might be your cause. The blend air door on the control unit (under dash assembly) could be stuck in the heat setting only (door facing up). Cool air will pass through that door and enter the heater core, warming up the temp.
Another factor could be your compressor clutch gave out. If you turn on your a/c and look under your hood where the serpentine belt is, there should be a pulley with a Y shaped object sitting 1/8" inch out from it, look to see if it spins or stays solid. If it turns when you turn on a/c, your compressor clutch is working. I think we use the wobble plate type rotary compressor.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille