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2004 Pontiac Grand Prix cooling fans will only run when sensor is unplugged have replaced sensor same result have checked fuses have checked fans initially started running a little warmer than usual replaced radiator due to sludge build-up change the thermostat change sensor still have same result help please


   
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Location: Oregon
fans turn on when the coolant temp gets too high. what temperature is your vehicle running at that makes you think there is something wrong? Does it go into the red?

your low speed fan should come on around 212F. high speed fan should come on at 221F. Turning on the ac should turn on the low speed fan at 180psi and the high speed fan comes on at 240psi.

if the fan does not come on in any of these situations, check the fan motor to see if it is good in low and high. If the fan is bad replace it. If the fan is good check you fan relays in the under-hood fuse block.


   
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Fan motors are good fuses are good when I plug the sensor into the sensor harness the fans won't work when I unplug the sensor I get the low temp fan temperature gauge reads at the three-quarter Mark I won't allow it to get to the red but it would if I let it


   
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you should use a scanner that can show you the data stream. the computer (powertrain control module) controls the signals to activate the fan relays. the scanner will show you the coolant temperature and the fan request for both low speed and high speed. you can watch the temperature rise and see if the fan request says yes and then check to see if the relay acts on the computer's signal. If the fan doesn't come on you can check the signals at the relay and see what is not working properly. You would watch the temperature all the way until the high speed fan should come on. That would still be in the safe range but be in the higher portion of it's scale.

check the temperature that the computer says it is vs, what the gauge says what the temp is. The stepper motor for the gauge could be off. did the hot coolant warning indicator ever come on?


   
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No no hot coolant warning and I'm going to try that thank you!


   
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One other thing when I changed sensors and plug the sensor up and drove the car maybe a mile the RPMs started fluctuating and it shut the car off I assume that was a protection mode do I need reboot the computer settings like you would your home computer when I change sensors and stuff out like that


   
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changing the coolant sensors does not need a reboot of the computer.


   
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