My 2000 Altima’s A/C has been running hot and cold. It will either start out cold and then get warm or vice versa. I took it to the Nissan dealership where they said they found a leak. The notes on the invoice say that they “replaced an O-ring from the A/C line to the evaporator and re-charged the system.” When I left the dealership it was blowing ice cold air. Then the next morning again it was blowing cold air as I drove to work, but later in the afternoon when I left work and I turned it on again it was blowing warm/hot air. I took it back to the dealership. On the whole drive to the dealership (about 20-30 minutes) it continued blowing warm/hot air. The car sat there overnight and they checked it again the next morning. The notes on the invoice read “Let vehicle run for approx 1 hour and found 41 degrees at center duct. Checked pressures with manual guages. OK during road test found center duct temp at 38 degrees.” Again after leaving the dealership it was blowing ice cold air on the way to work, but then when I drove home that afternoon…warm/hot air again. I’m going to take my car back again tomorrow. So what is wrong with my car? Are the technicians missing something? I’m almost starting to believe that it has to do with turning the engine on and off. Because it will be fine during one trip but then if I stop somewhere (like grocery shopping) when I turn the A/C back on it’s blowing warm/hot again.
Not an expert but the first thing is to see if you hear the air conditioning compressor running when it blows warm air. When it blows cold, you will hear it cycle on for some seconds and then turn off, then on, then off. If it still does that when it blows warm air, it would tend to suggest it is low on refrigerant still. If it does not go on when it is blowing warm air, then the problem is the compressor it not being turned on. So that could be the clutch, the switch for it or your AC controls, a faulty pressure switch, etc. If you tell them the compressor doesn’t cycle when it blows warm air, they’ll head down a different path than they have been.
Also, there is a “blend door” in the heater/A/C system that controls airflow through the ductwork. This door decides whether you get hot air or cold…Those controls should be checked too…
When you state the air becomes warm, does this happen immediately after starting the car and operating the A/C or does it become warm after being in operation and working fine for a bit; say 10-15 minutes?
Both situations happen. Sometimes it will start off warm and start to blow cold after about 15-20 minutes. Other times it will start cold and if it gets too cold and I turn it off and then turn it on again (maybe after about 15-20 minutes) it will blow warm.
This rules out an evaporator freeze up anyway.
The next time the A/C is not blowing cold, raise the hood and look down at the center part of the air compressor clutch/pulley.
With the A/C on, the entire assembly should be turning. If it is not, then you have an electrical problem in the circuit.
As to where, that’s hard to say. The problem could be a faulty clutch coil, faulty wire connector to the clutch even, faulty relay, or even a defective pressure switch.
Sorry I can’t be more specific, but if the problem is electrical in nature it really should not be that difficult to trace down. With an inoperative compressor it should be a simple matter of working from the clutch coil backwards.
Also, when the A/C is on make sure the radiator fans are working. If I remember correctly this model uses a dual pressure switch that will cut the compressor off if the pressure drops too low or a cooling fan is not working.
Hopefully, the system was evacuated and recharged properly. Any thing done haphazardly during this could also cause your problem.
It would help to know the high and low side pressures, but I assume they did not write this down?
I took my car back again and now they say I need an “air conditioning thermo control amplifier switch” What exactly does this do and I’m wondering why it took them 3 separate visits to find this out. I told them that it seemed that the A/C would blow cold air early in the morning when the outside temperature is cool but it would start blowing warm air when it sat out in the sun all day.
I’m afraid that I do not know exactly what that part is and without a wiring schematic it’s hard to make a guess.
Many Nissans used a thermostatic switch to control compressor operation and at one time (prior to your model of car as far as I know) they did have some problems with those switches.
I’m just not sure if I’m on the same page as they are.
Sorry I can’t be of more help on this issue. Since it’s an on/off thing and obviously electrical related about all I can do is go along with their diagnosis for now.
Will do some digging on the net and see if what I’m thinking of is the same thing they state is bad. 
http://autos.msn.com/research/vip/Reliability.aspx?year=2000&make=Nissan&model=Altima
This link shows that MSN’s auto rating system list the thermo control amplifier as a “known” problem with the Altima.
My 2000 Alty is going under the wrench tomorrow to have just that part replaced.
All the mechanics who have looked at the car have been dumbfounded and couldn’t find a problem. They just wanted to start replacing parts.
I went the the Altima owners’ and Nismo sites and there are numerous references to this same problem.