Question

Car fans won't run untill i turn on ac

There's a 1999 Pontiac Grand Prix a family member of mine owns and we've been trying to get it working for a trip he's going on. My main question is how we would get these fans working without him having to keep the air on?
We've changed the thermostat, the sensor, and while looking at the fuses I found this car has five fuses to control the fans. He bought one of the fuses and replaced it and the fans still wont run without the AC. Should we replace the other two that might be bad? I've called the part houses and they each tell me they don't have the other fuses. Where can I get the other fuses? Could there be anything else wrong besides the fuses?

Any help and all help is welcomed
Thanks to all that helps.

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Answer
This is usually that coolant temp sensor issue.

Here's what others have written: "The coolant temp sensor is at the bottom of the radiator."

At that many years crud can accumulate and insulate the temp sensor. You may have pump and other issues as well but the cheapest thing to try is the most aggressive radiator flush you can find. Shops used to have a machine to do this but I haven't seen one in years.

If you have the time, here's the full rundown of how to test this part:
https://axleaddict.com/auto-repair/Coolant-Temperature-Sensor-Test

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Answer
Common temp sensor issue

We have dealt with this kind of issue before and it is more likely the temp sensor acting up or needs a replacement. We've also seen a fan issue turning slowly and causes almost the same issue but this rarely happens as this fan mostly either working or busted.

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Answer
Coolant temp sensor issue test to detect the issue root

The coolant temp sensor is at the bottom of the radiator. If that much crud came out, you may need to have the radiator removed and run through an ultrasonic bath to get all the crud out. It is probably insulating the sensor and blocking critial flow. In the case it is resulted from Coolant temp sensor, the next thing you should do is to test it, here is the step-by-step guide: https://carfromjapan.com/article/car-maintenance/headed-test-coolant-temperature-sensor-heres/
In case you find it to be faulty, you must replace it at the soonest.

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Actually, there may be two such sensors

One will connect to the temp gauge on the instrument panel and one may be in another location that sends this information to the car's computer to determine its warm-up state. A proper or false reading on the instrument panel does not rule in or out that there's no problem with the other.

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