I have a 2006 3.9 L GTP. I have an issue with diagnosing why my engine is overheating. The cooling fans only cycle on when the air conditioning/defrost is turned on. All the fuses have continuity and I've replaced the thermostat and engine coolant sensor. Also... the temp gauge never exceeds normal operating temperature even when the engine overheats. I know the engine overheating because the coolant is boiling in the reservoir and steam is escaping out the overflow tube. Any ideas why the fans are not cycling properly? I seem to remember hearing about a service bulletin and a computer software ssue.