Overheating Coolant temp climbs at low to moderate vehicle speeds going up grade. Cooling fans begin to run when temp gauge reaches center of temperature range. The thermostat and radiator have both been replaced within the last year. I found nothing blocking cooling air flow, but I suspect there may be inadequate air flow due to improper fan operation. What could cause the described issue? Thanks in advance.

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.

Both radiator cooling fans are running all the time ? Key off ? The cooling fan opperation
HOW they work
Cooling Fan Control
The engine cooling fan system consists of 2 cooling fans and 3 relays. The relays are powered by the battery positive voltage circuit and controlled by a switched ground from the powertrain control module (PCM).
During low speed operation, the PCM supplies the ground path for the low speed cooling fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan low relay coil, closes the relay contacts, and supplies battery positive voltage from the COOL FAN LO fuse through the cooling fan motor supply voltage circuit to the cooling fan. During high speed operation the PCM supplies the ground path for the cooling fan low relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cooling fan high relay and the s/p cooling fan relay through the high speed fan relay control circuit.
The PCM commands the fan on under the following conditions:
• Engine coolant temperature exceeds approximately 98°C (208°F) Low Fan Speed
• Engine coolant temperature exceeds approximately 102°C (216°F) High Fan Speed
• A/C refrigerant pressure exceeds 361 kPa (52 psi) Low Fan Speed
• A/C refrigerant pressure exceeds 2100 kPa (300 psi) High Fan Speed
• When the engine coolant temperature exceeds 112°C (234°F) at key off, the fan high speed will run for up to 300 seconds. If within that time frame 102°C (216°F) is reached then fan speed will change from high to low speed. If within that time frame 99°C (210°F) is reached then fan speed will change from low to off.
The PCM commands the fan off under the following conditions:
• A/C is requested and engine speed exceeds 6,240 RPM
• Engine coolant temperature exceeds approximately 99°C (210°F) turns the cooling fans from low to off.

sounds like one of the relays are stuck closed .

Cooling Fan Always On
Step
Action
Yes
No
Schematic Reference: Engine Cooling Schematics , HVAC Schematics in HVAC Systems - Manual
Connector End View Reference: Cooling System Connector End Views
DEFINITION: One or both engine cooling fan motors run continuously in high or low speed.
1
Did you perform the Diagnostic System Check - Vehicle?
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
Install a scan tool.
Turn ON the ignition, with the engine OFF.
With a scan tool, command the Fans Low Speed ON and OFF.
Are one or both cooling fans ON?
Go to Step 3
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
3
Are both cooling fans running continuously?
Go to Step 4
Go to Step 6
4
Are both cooling fans running continuously in high speed?
Go to Step 5
Go to Step 7
5
Test the low reference circuit of the A/C refrigerant pressure sensor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 18
Go to Step 14
6
Important: The S/P relay is located under the front bumper on the drivers side of the vehicle.
Remove the cool fan S/P relay.
Did the right cooling fan turn OFF?
Go to Step 10
Go to Step 8
7
Remove the cool fan LOW relay.
Did the cooling fans turn OFF?

Have the vehicle checked for DTC'S - diagnostic trouble code's if you haven't already . The vehicle has two cooling fans an three control relay's that are energized by the PCM - engine computer . Coolant temp sensor tell's the PCM engine temp.
Do you know what a wiring diagram is ? Free one's here http://www.bbbind.com/free-tsb Enter vehicle info. year make , model an engine . Under system click on engine ,then under subsystem click on cooling fans . Click the search button then the blue link. Watch this video.
Relay Driven Dual Cooling Fan Diagnostics

Also ,what are you going by to say it's overheating ? Temp gauge in the instrument cluster ? GM had problems with Stepper motors inside the instrument cluster . 2003 - 2006 . Hooking up a scan tool that can view data would show what the coolant temp sensor is tell the PCM .

Using a infered temp
This would show the real temp of the engine.

DTC P0480 or P0481
Battery positive voltage is supplied to the cooling fan 1 relay from the COOL FAN #1 fuse. The powertrain control module (PCM) controls the cooling fan 1 relay by grounding the low speed cooling fan relay control circuit via an internal solid state device called a driver.
Battery positive voltage is supplied to the cooling fan 2 relay and the cooling fan 3 relay from the COOL FAN #2 fuse. The PCM controls the relays by grounding the high speed cooling fan relay control circuit.
When the PCM is commanding a relay on, the voltage potential of the control circuit should be low, near 0 volts. When the PCM is commanding the control circuit to a relay, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the DTC will set.
The PCM will monitor the control circuit for the following conditions:
• A short to ground
• A short to voltage
• An open circuit
• An open relay coil
• An internally shorted or excessively low resistance relay coil
When the PCM detects any of the above conditions, the DTC will set and the affected driver will be disabled.

I would say the instrument cluster is probably the problem . If the fans come on when A/C is turned on the system is working.

How to repair GM Instrument Cluster Speedometer Gauge Stepper

A pontiac montana ? There are three relay's that control your cooling fans ! unplug what sensor ? The engine coolant temp. sensor is an input to the PCM - engine computer . When temp. reaches 213 degrees the PCM energizes the relays , depending on low or high speed . Using a scan tool to see if the PCM will turn on the fans an checking engine coolant temp.data is the right way to diagnose this . Relay Driven Dual Cooling Fan Diagnostics

It is normal for a temperature gauge to fluctuate when the car idles, you will notice 20 to 50 degrees bounce. As long as it is not going into the red or blowing steam don't worry about it.

That is normal cooling fan operation. the fans are computer controlled and not turned on until the engine reaches the pre programmed temperature, that is about 185-195F

Try this for a diagnostic:
Check coolant level. Fill if necessary. Check for leaks.
Start vehicle. Allow to run.
Observe coolant temp gauge. It should slowly climb to center.
Once coolant temp gauge reaches the middle, the coolant fan is triggered to activate by the vehicle's computer (ecm). Fan should come on until temp drops slightly then cycle off.
If fan is working, move to next step. If fan isn't working, possible causes:blown fuse, faulty fan relay, coolant fan switch, faulty cooling fan.
Next step.
If there are no leaks, and the fan works properly as described above, have a compression test performed. This test will basically measure the compression power (in PSI) in each cylinder by screwing a device into the spark plug hole of each cylinder cranking the vehicle (with ignition disabled) and observing the gauge. Each cylinder reading should be significantly close to its next one. If not, suspect a faulty cylinder head gasket.

Instrument cluster - GM had problems with a part inside the cluster , Stepper motor , These faulty step motors caused the gauges to act erratically . 2003 to 2006 ! ALL GM vehicles . Just replaced a cavalier instrument cluster for what customer thought was a over heating problem .The fans come on low speed when engine temp. reaches 220 degress
The PCM commands the low speed cooling fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 104.25°C (220°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 min. regardless of coolant temperature.
The PCM commands the high speed fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 107.2°C (225°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 insures idle stability by preventing the fans from cycling between high and low speed.

The cooling fans on this vehicle won't come on unless the a/c is on or until the engine coolant temperature reaches about 220 degrees. Once the coolant has reached this temperature the fans will come and and run until the engine cools down to about 210 degrees and they will then turn off until the temperature climbs up again. The fans usually aren't on at all at highway speeds because enough air is being forced through the radiator to keep everything cool. The true test of cooling fan operation is to allow the car to idle in park with both the a/c and defrost turned off and let it run until the fans come on. When The temp gauge reaches about 3/4 of the way up, give or take a bit, the fans should come on. Keep an eye on the gauge so the engine doesn't overheat just in case there really is a fan control problem but I'd say you'll most likely find that the fans are working properly since they do come on for the a/c. Hope this helps!

When your engine is operating normally, the temp on the gauge should be between 190 and 225. The thermostat is set to open at 190 and this controls minimum engine temp. If you sit in traffic the temp will climb to 225 and then the cooling fan will begin to operate to make sure that you don't overheat.You know that water boils at 212 but you have to add 10 degrees for every pound of pressure the rad cap is set to. Also your coolant has antifreeze in it which raises the boiling point even more. A normal overheat condition of your engine would be 300 or more. Does this help?