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The 3 blinks usually means something isn't in the correct position for the A/C to operate- either the mode control is in the wrong position (a/c doesn't work in defrost mode, etc.) or the compressor won't turn on for some reason. There is another reset procedure other than the one Nathanjax mentioned- first start it up and set all hvac controls to off, then shut off the engine, disconnect the positive battery cable, wait 10 minutes, then before reconnecting it touch it to the negative battery post for a complete discharge of the system. Reconnect the cable and start up the engine but don't touch the hvac controls for at least 5 minutes during which time all the actuators (mode, blend, recirc) recalibrate. After 5-10 minutes, try the a/c. This is the only thing that has worked for me in the past after I changed out parts of the system.