Cooling fan running but not full speed what seems to be causing this

Do the radiator cooling fans work ? With the car idling turn on the A/C , does the radiator cooling fan come on ?

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

Try unplugging the cooling fan relays. if the fans stop running, replace the relays. The fan relays are located in the plastic box under the hood, near the battery on top of inner fender well. The box lid has fuse and relay description printed on it.

I would say more than likely it the radiator cooling fan. For some reason some vehicles, no matter what temperature, the cooling fan clutch engages which makes the cooling fan run at full speed like there's no fan clutch.

It sort of makes it sound like the engine is revving up but it's simply the sound of the rushing air from the fan clutch making the cooling fan spin at full speed.

Hope this helps.

Temp sensor or fan control switch or the A/C is engaged causing the fan to run at high speed

Hi Dean, I don't really know what you mean by the register, but I believe your problem is to do with the resistor which is built into the fan housing. It consists of three wire wound resistors and is fitted into the fan housing to take the benefit of the fan to cool the resistor pack. The climate fan at full speed runs directly from the battery voltage. The lower speeds run through the relevant resistors (which are of different values) and thereby run the fan at decreasing speeds. Remove the covers from the both sides of the fan housing and on one side you will see a small panel held in position be one or two screws. The panel will have cables plugged into it, which are those for the fan. To confirm you have the correct panel, turn on the ignition and the fan at full speed and disconnect the plug from the socket. the fan will stop and bingo! you have it! Remove the unit and either repair the unit or better still go to a breakers yard and buy a replacement from there before they wreck it! Fit it in, connect the plug, put back the paneling and enjoy. Regards John

Engine quickly overheating is often caused by a stuck closed thermostat. However, in your case since the fans don't come on, the most likely culprit is a bad fan speed controller. These are variable speed fans controlled by the speed controller. They fail when rust forms on the mounting/cooling area where the controller is mounted. This is an electronic device that has a heatsink on the back. When rust forms it can't dissipate heat and that burns out the controller. See the post below for more information on how to replace it.
Car overheats Chrysler Vehicles Ricks Free Auto Repair Advice

you may find that the cooling fans only run at half speed when the a/c is on they might be set that way & run at full speed when the engine starts to overheat (in stop & go traffic) the a/c may need regassing that's why it is not cooling properly.....hope this helps...... cheers..

The shroud around the radiator fan mainly assists cooling at lower road speeds - it increases the efficiency of the cooling fan/s.
At higher road speeds the increased airflow due to vehicle speed makes the fan and shroud less important in that circumstance.

Running hot at higher roadspeeds can be caused by either engine mechanical fault (blown headgasket etc) or defective cooling system:-
Check for combustion gas bubbles in coolant reservoir - and ensure engine has no other mechanical running problems.

Check cooling system as follows:-
1) Faulty thermostat,
2) Blocked radiator air flow due obstruction such as grasshoppers or other insects,
3) Blocked radiator core (internal) by sludge/scale etc.
4) Collapsing radiator hose on intake side of waterpump (due to softened hose),
5) Cooling system not properly filled with coolant due air locks in the system - bleed off any air (as manufacturer specifies) during re-fill with coolant and ensure flow to heater core is enabled.

check for leaves obstructing radiator and between radiator and ac condenser. radiators on these vehicles are a bit small making every inch valuable cooling surface. I would also consider using some radiator flush to clean the inside of radiator core.
Additionally, be sure that your cooling system is full., running a bit low can also cause overheat.
cooling fans could actually be removed at speeds over 60...The air going through radiator at speed is likely enough to keep cool.