Possible Causes

WARNING: This page is about a different car, the 2005 Mercury Mountaineer and 2005 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.

An open in circuit or 331 (PK/YE).
A short to voltage in circuit 321 (GY/WH), 348 (VT) or 331 (PK/YE).
A short to ground in circuit 331 (PK/YE).
PCM.
(DATC systems) DATC module.
(Manual climate control systems) Function selector switch.
A/C compressor clutch field coil.
A/C control relay.
A/C clutch air gap.