How to tell when you have a bad fan clutch

Smart man using a scanner for the actual temp. Assuming you're not losing coolant....2 things I'd look at here, 1st is you may have air in the system, make sure to purge all the air. 2nd is your fan clutch, I've replaced a few fan clutches in my own Trailblazer. Have had them bad right out of the box brand new. Have had bad brand new fan clutches on 2 of my last Trailblazers. You should hear it engaging especially in stop and go traffic.

It is a difficult process to check the correct operation of a thermal fan clutch.
Mostly if it is suspected the clutch is not working as it should and the age and the mileage of the car is significant it is best to replace the unit.

The clutch would be suspected if strange noises are heard that can be traced to the fan or if some engine overheating is being experienced, mostly at low speed or extended periods of idling.

To check the fan meets specification, first you need to know the temperature the clutch is designed to fully engage and then by whatever means available the fan clutch must be raised to that temperature and force applied to ensure direct drive has been achieved.

First things first....
INSPECT COOLANT LEVEL
Low ????
PRESSURE TEST SYSTEM
PRESSURE TEST RADIATOR/EXPANSION TANK CAP
INSPECT DRIVE BELT(s)
INSPECT COOLANT HOSES
INSPECT HEATER HOSES
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES
TEST THERMOSTAT OPERATION
INSPECT RADIATOR TEMPERATURE
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

Begin with the basics....
INSPECT COOLANT LEVEL,
Low ????,PRESSURE TEST SYSTEM,
PRESSURE TEST RADIATOR/EXPANSION TANK CAP,
INSPECT DRIVE BELT(s),
INSPECT COOLANT HOSES,
INSPECT HEATER HOSES,
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES,
TEST THERMOSTAT OPERATION,
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION,
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI,
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION,

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

Its very possible the fan clutch has locked up and is now a direct connection from the fan to the water pump.
Depending on if it is controlled by temp or speed, it may not run at full speed when cold or at idle.
When the engine is cold and not running, a normal fan clutch would let you turn the fan blade without turning the pulley.

This can be caused by a bad controls or bad blend door motor, the door motor and controller will need to be tested to see which is causing the problem,

when you say temp fluctuates what happens? are you using the a/c and you turn temp down one click and it goes to hot ?
Is the temp going from hot to cold hot to cold as you drive?

Hi,
A/C increases Engine Temp in two ways.
1, extra 2-6hp load on engine.
2, hot condenser in front of radiator heating air going through cooling system.

A small difference in engine temp can be expected, but if the car is running hot get it sorted out.

250 DEGREES ENGINE GETTING TOO HOT.SOUND LIKE CLUTCH FAN FREEWHEEL.IF CLUTCH FAN DONT LOCK IN OR HAVE RESISTANCE TURNING BY HAND WHEN ENGINE HOT.FAN CLUTCH IS BAD.IF FAN CLUTCH OKAY COOLANT LEVEL CORRECT.I WOULD CHANGE THERMOSTAT FLUSH OUT ENGINE AND RADIATOR.IF ENGINE TEMPERATURE STILL READS 250 DEGREES OVER FLOW JUG OR RESERVOIR NOT BOILING OVER THE COOLANT TEMPERATURE SENSOR FAULTY.

There may be an obstruction in one of the hoses, there may be an air pocket in the system somewhere.
Are you sure there is actually an overheating going on? Maybe it's a bad temperature sensor or temperature gauge stepper motor.

I don't have the solution but I have the same problem. You are not alone. I have done everything you have done and have run out of options as far as fixing it with OEM parts. I am going to over ride the system with a hot wire and toggle switch and tie into the led wire on the #1 fan so when temp starts to rise past 180 degrees I can turn the fans on. Freeway driving is ok for me, no fan needed, just when I get off the freeway and around town engine gets hot. My toggle switch has a L.E.D. light in it to indicate the switch is on.