This 2000 Nissan Xterra came in with the complaint that the a/c works good sometimes but it may not work at all at other times. The easiest place to test is at the thermistor lead connector (white) at the bottom left of the glove box.
It can be done without removing the glove box but it is easier with the glove box assembly out of the way. There are two screws located in the lower corners. Two screws located in the upper corners and two screws located in the upper center at the latch bracket.
The testing is as follows. Key on and engine running with the a/c turned on: Green/orange wire will have battery voltage from fuse #29 in the underhood fuse box, the Blue wire will have battery ground from the a/c control assembly and the Blue/black wire will have a nominal 4-5 volts with the thermistor off (temperature below 40 degrees F) and less than .5 volts with the thermistor on (temperature above 45 degrees F). The thermistor assembly applies a ground to the circuit which is sent through the dual pressure switch and then on to the engine control computer. This one failed the test and needed to be replaced. The proper way is to remove the refrigerant and then the evaporator case but I prefer a simpler way. It can be a little painful but it only takes about ten minutes from this point to change it. There are three phillips headed screws that have to be removed along with three 10 mm headed screws as shown in the following pictures.
Once the screws are removed, the two halves of the evaporator case can be separated enough to look in, see the thermistor, carefully and a little painfully insert your hand into the box and removed the sensor. Slide the new one in, in the reverse order and install the previously removed screws. The long tab pointing down slides into the evaporator fins and tab pointing to the right is the actual thermistor bulb.
I was going to replace the fresh air filter but after opening the evaporator box there is no filter that I can see is it somewhere else?I touched the evaporator and it feels oily and coated with dirt
On this particular vehicle there was no cabin air filter either. If the evaporator core is dirty to the point that air will not flow through it then it will need to be cleaned. If you have some other concern I would need to know more details about the issue in order to help.
The only idea I have is to get a voltmeter and do some testing or take it to a mechanic.
I have not done any test. I just happen to notice today that The button you depress for A/C doesn’t lite up. Seems like it use to though, when it was still blowing hot air. Any ideas?
What tests have you done and what is your data?