2005 chevy malibu cooling fans not coming on causing the car to over heat at idle. they will come on if you turn the a/c on and it will not over heat. if you unplug the coolant temp sensor both fans come on. does anyone have any cue what might be going on

Is the engine really overheating ? Hook up scan tool an see what temp. sensor is telling PCM / ECM . Did you use Infrared Thermometer Temperature Gun to see real temp. of the engine ? GM vehicles from 2003 to 2005 had issues with bad stepper motors in instrument clusters ,could show over heating when is really not . https://www.youtube.com/watch?v=oTm2oDx-_GM&t=30s

replace the temp sensor its gone out of range
service the fans
these fans have carbon brushes maybe one is a bit worn set or something.!!!

If your talking about the cooling fan for the radiator, if it's an electric fan then it is controlled by a temperature sensor that's screwed into the block (usually near the thermostat housing) the fan shouldn't come on right away, until the temp is reached, then it should turn on & run for a while til the temp drops again,then it will turn off, then repeats itself. If this is what's happening then that's normal, if not check the temp sensor.
Now if it's a clutch fan (not electric) then the clutch that's bolted to the fan could be your problem. hope this helps.

It is normal for the A/C compressor to come on during defrost mode. This is done to prevent provide dry air to the windshield to prevent fogging when the defroster is turned on. The blend door is designed to allow heat to the windshield, but also some of the cool air from the evaporator to prevent fogging.

Thermostat is not working. Replace it and you should have hot/warm like it used to.

Blow out the radiator core from the engine side with compressed air to remove bugs and other debris. Make sure the cooling fan comes on when the engine gets to operating temp. If the car car has AC, turn it on which should start the fan immediately. If no fan action, try to turn the fan blade(s) which should turn easily with no noises. Check the fan fuse and fan relay. If the fan(s) come on fine, check the coolant level in the radiator core--it should be right at the top. Check the seal on the cap and replace it if it is decayed or damaged. Make sure the reservoir bottle is filled to the line and the connecting hose is in good shape. It is possible that the thermostat is sticking and not opening fully. Also, check the intake and outlet hoses to and from the radiator for firm condition without decay, collapse, or other damage. A hose can sometimes peel away a portion of the hose interior which can cause blocking. Hope some of this helps!

Do your rad fans kick in when it gets to about 100 degrees?

is electric radiator fan coming on? if not the relay or fuse is bad

On some cars the fans will run anytime the heating or cooling system is in the defrost mode try turning it to the vent mode or floor and see if one or both go off

your coolant temp sensor on the engine block is probably faulty, however, keep in mind that if it really isn't all that hot outside they may not need to turn on. Once you activate the A/C the fans come on automatically since they have to cool the condensor...

If your car is not overheating do not worry about it.