2004 Colorado cooling/heating fan will only run on high speed

if the scan tool ECT sensors show 180f to 200f it is not overheating
what are you talking about.
blower dash fan.
engine RAD fans < guess only this.
or front AC condenser fans.
overheats parked
overheats driving yes or now fast?
oveheats only AC on, none of this told an matters HUGE.
learn to use a scan tool?
you can not fix any modern car lacking one. $100 buy one.
here are the proofs of over heating. al or most.!!!
dash gauge pegs, see that can be coolant low, easy no>?
scan ECT shows temps way over 200F, 265 is boiling point.
coolant on the ground AF 50% green,yellow orange pink what eva.
coolant on the passengers toes(foot well heater core leaks)
coolant in oil pan , and dip stick end drips, chocolate milk.
engine pings, or under load (hills) dont let it. pings is too hot. or dead EGR.
huge engine power loss from ping. (and lots more)
engine guts make huge nasty metallic sounds. bad that. check oil levels.
dash gauge engine heat PEGs , can mean overheating.
dash gauge tad high is normal. z(operators guide said so)

the service manual tells you clear as day what temps trip the fans on if you just RTM/ 214 to 220f , if ECT show that the fans run in that range and lacking a scan tool you will NEVER KNOW IT.
'the dash gauges is NOT and instrument it is a just a crude, toy gauge, the ECT is what matters period. or IR thermal gun pointed to thermostat housing then to top and bottom RAD tanks,


this magic tool and skills I can find dead RAD tubes in 1min flat
love mine, Have 2.

shows 170 here, or 180f if drive 5 miles. your car is 200f stat
a parting shot
no 19 year old thermostat run that long.
the top usa maker claims a 7 year design life
I bet yours is bad. as the wax pellet ages in it ,it shrinks and the TEMPS go way way too high, or are sluggish.
seen it vast times and is frankly normal service.

What makes you thing it's a sensor ? A lot of thing's could cause this , taking the vehicle to a qualified repair shop is what should be done. They will do a full system scan .
ENGINE HOT A/C OFF
Cooling System Description and Operation

Coolant Warning Messages
The radio will display the following messages if the following conditions exist in the cooling system.
• Engine hot--A/C OFF will be displayed if coolant temperature is above 117°C (243°F). For imports 115°C (239°F).
• Engine coolant hot--Idle engine will be displayed if coolant temperature is above 118°C (245°F).
• Engine overheated--Stop engine will be displayed if coolant temperature is above 123°C (253°F).

Cooling Fan Control - Two Fan System
The engine cooling fan system consists of 2 puller type electrical cooling fans and 3 fan relays. The relays are arranged in a series parallel (S/P) configuration that allows the engine control module (ECM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block. The ground path is provided at G104.
During low speed operation, the ECM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the low speed fan relay coil, closes the relay contacts, and supplies battery positive voltage from the low fan fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan S/P relay and the right cooling fan. The result is a series circuit with both fans running at low speed.
During high speed operation the ECM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. After a 3 second delay, the ECM supplies a ground path for the high speed fan relay and the cooling fan S/P relay through the high speed cooling fan relay control circuit. This energizes the cooling fan S/P relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time, the high speed fan relay coil is energized closing the relay contacts, and provides battery positive voltage from the high fan fuse on the cooling fan motor supply voltage circuit, to the right cooling fan. During high speed fan operation, both engine cooling fans have their own ground path. The result is a parallel circuit with both fans running at high speed.
The ECM commands the low speed cooling fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 94.5°C (202°F).
• A/C refrigerant pressure exceeds 1447 kPa (210 psi).
• After the vehicle is shut OFF, if the engine coolant temperature at key-off is greater than 101°C (214°F), the low speed fans will run for a minimum of 60 seconds. After 60 seconds, if the coolant temperature drops below 101°C (214°F), the fans will shut OFF. The fans will automatically shut OFF after 3 minutes, regardless of coolant temperature.
The ECM commands the high speed fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 104.25°C (220°F).
• A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).
• When certain DTCs set
At idle and very low vehicle speeds the cooling fans are only allowed to increase in speed, if required. This ensures idle stability by preventing the fans from cycling between high and low speed.

DTC P1258: Engine Coolant Overtemperature - Protection Mode Active

Check the bearings. If the friction is high the fan will not run at rated speed. When it runs slower it uses more current and gets hotter...

Disassemble, clean and lubricate to make sure fan runs full blast and does not burn out.

Or if the fan has two windings, one for starting and one for running it is possible that the high speed run winding has some shorts in it resulting in more current and more heat.

Are you talking about the radiator fan? How hot does the engine get in degrees F? Some makes and models, the radiator fan won't turn on till about 220 degrees F. Check for applicable trouble codes, see if any code that refers to the radiator fan circuit or coolant temp sensor? Some of the national brand auto stores will check codes for free.

Of course, as far as overheating in general, you have the thermostat--water pump and other possible issues. Are you losing coolant?

sounds like a bad blower resistor.The switch runs a straight 12 volts to the motor in high,but runs through the resistor on the lower speeds.The resistor is located somewhere on the HVAC unit(it requires airflow to avoid premature burnout).

The resistor needs to be replaced, common on the Colorado's
http://coloradofans.com/forums/viewtopic.php?f=8&t=48293&p=1038821&hilit=resistor#p1038821
 

The fan speeds are usually 99% corrected by changing the fan resister board....usually near the fan motor under the dash in front of the pass seat.Cost .......around 30$ for the board.You can do it.
Follow the wires from the fan motor to the board connector.

I agree with philip, i have not studied the electrical system. but to my best guess the electric fans are tied into the engine cooling system. when the engine is not requesting the fans to come on. then their is very little air passing threw the cooling package. When their is no cool air passing by the condenser this will not draw the heat out of the refrigerant leaving excess heat in the system and when the cab fan blows this is were the excess heat is getting dumped. then at higher speeds their is enough air passing threw the condenser and radiator to cool them both.

You need a resistor pack, which is likely located in the air box, near the blower motor.

most of the time these have a fan speed resistor in the cooling fan box under the dash there in there to keep them cool due to a lot of heat going through them when there on 1,2,3 and sometimes thay give up. the reason it runs on 4 is that its the highest setting and is getting the full 12 volts and does not run through the resistor. a replacment resistor should get all your speeds back