Hello, there are only 2 reasons the fan will come on high,
- the engine control module is internally shorted
- the fan control module is internally shorted
the only test you can do without a scanner is to test the signal wire from the ecm to the fan to see if it has power, if the wire does have power that means the ecm is activating the fan , in that case you need to get a scanner to see if the ecm is wanting to activate the fan or not. if it is not wanting to activate the fan then the ecm is bad. if it is wanting to activate the fan then further diagnosis is required to see if the air conditioning control module is telling the ecm to activate the fan .
now if the you test for power on the signal wire and you get no voltage that means that the fan assembly is bad. the wire you need to check for power on, is on the big connector at the fan control module assembly.i will attach a picture of the location.
the yellow wire with the blue stripe is the ground signal to the fan so you want to use a meter and test from that wire to a positive source.
hope this helps. the fan module is the silver box in the picture which is under the passenger headlamp. you should be able to access it from under the bumper.
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