Did you change the water temp sensor or the air intake sensor? Neither one should effect the A/C coming on or not though. However, your blower motor may be shorting out. Do you feel cold from the vents but just no air flow? Kick the right side of the driver's footwell towards the bottom. I got my daughter's blower to start up that way until it completely failed. Had our mechanic friend install a new blower (there's a DIY link under Resources at top right of this page) and it works like a charm now. There's also a linked fuse along the top left of the driver's side under the hood that goes to the blower motor and contacts could be corroded. It's rectangular and the cap pops off easily.