Found this post searching in the forums. Maybe it will help.
Heat/Cold Blend Door
Pull the panel off right drivers side footwell. It comes off easily as it just uses spring clips to secure. There are two similar black housings with wires running to them, they are the servos for your hvac controls. The bottom one mixes heat and cold air. With the ignition on, turn the heat/cold rotary knob and see if the servo is working. If it is not, you have a servo issue: bad servo, blown fuse, wiring issue. If it is working, remove the two screws holding it in place and see if the shaft to the blend door is broken. If the shaft is broken, it will be major $$ for the dealer to fix it. Mine was broken and I didn't want to spend over a grand to have the dealer repair it. I used a piece of brass bushing with the correct i.d. to use as a coupling. Use some two part epoxy, the coupling bushing and glue the thing together in the correct position. My repair works great and it cost about ten bucks versus the $1200 the dealer wanted. Look at my post for the heater blend door repair in this section of the forum. I've added a couple pictures. They are not great but should give you an idea of what to do.
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.
The only thing I can add is that my dads heat in his f150 wasn't working. Our mechanic said the blend door actuator is probably broke. Well he was right. What we did now that it is winter is stuck a screw driver up into where it opens and messed with it until the heat worked again. We cant use the knob though. It was just a temporary fix for winter
If you by chance have access to a UV light (black light), get a long extension cord and you can inspect for leaks. Most common r134a systems have today a dye built into the fluid. I would start off on the high pressure side (hot) from the compressor and work your way to the condenser to the H Block (TXV is inside and it converts hot to cold), work your way on the low side to the evaporator, then to the muffler (its called that and it looks like a funny bell shape close by the compressor). I would hold the UV light roughly 2-3 inches away from the tubing.
Another method is a sniffer, but a mechanical shop will have access to that. I wouldn't recommend it at all cause it beeps all the time. It is supposed to detect r134a fluid to beep.
Your compressor clutch fluid should be PAG oil, but I am unsure of its number 2,4,8, etc. That's whats used to lubricate the compressor. The HVAC under dash is a one piece unit and a pain to take off cause its literally between the firewall and above your feet inside the cabin, so I would hope the blend-air door is good. I don't know if our cobalts use mechanical cables to move the door or its electronical. Coby would know that one
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