Where is the coolant temperature sensor located not the sender unit on the back of engine

the most expensive part of 3 possible replaced 2 times.
wow.
the sensor or the wires (end to end) are bad. (just one gauge fails?)
the service manual covers how this work.

I guess you meant the IP cluster engine temperature meter reads wrong and the engine thermostat is not bad, 23 years old and thermostats have design life of 7 years../
a scan tool set to scan PID, ECT sensor tells what coolant is doing.
189f to 213F is spec, temp range

the gauge coolant temp has:

Water Temperature Indicator Sender Unit
When the engine temperature is low, the resistance of the water temperature indicator sender unit (10884) is high, thus restricting the flow of current through the gauge and moving the pointer only a short distance. As the temperature of the coolant increases, the resistance decreases, allowing more current to flow through the gauge and resulting in a corresponding movement of the pointer.

these year trucks the gauge sender is not the ECU ECT sensor
and may be single wire sender.. try mostly fail this old, not the gauge
the wire on the sender can rust or even fall off.

Not low input sensor...P0117 KIA - Engine Coolant Temperature Sensor 1 Circuit Low Input

Possible causes
Low engine coolant level
Faulty Engine Coolant Temperature (ECT) sensor
Faulty engine coolant thermostat
Engine Coolant Temperature (ECT) sensor harness is open or shorted
Engine Coolant Temperature (ECT) sensor circuit poor electrical connection

Just replace coolant temperature sending unit....

At the rear of the right cyl head

possibly change the wrong unit
gauges work from a sender unit not a sensor
it is located next to the thermostat housing normally and has only 1 wire connection
they work by having power through the gauge to the sender through variable resistance that is affected by heat to ground
test disconnect the wire from the sender
turn on the ignition key , ground out the wire and the gauge should go straight to full hot
sensors on the other hand have more wires and relay the coolant temp to the ECM so that air/fuel ratios can be adjusted , and fans switched on as required

Not sure of your engine size so please pick the appropriate diagram:

Coolant temperature and oil pressure senders-4.3L engines

Coolant temperature and oil pressure senders-5.0L and 5.7L engines

Coolant temperature and oil pressure senders-7.4L engines

Coolant temperature and oil pressure senders-6.2L and 6.5L diesel engines

Water Temperature Indicator Sender Unit
The water temperature indicator sender unit is installed on the engine into an engine coolant passage. The water temperature indicator sender unit reacts to the engine coolant temperature. Its resistance decreases as the engine coolant temperature increases.

Water Temperature Indicator Sender Unit Removal
CAUTION: Do not mix Standard (green) Coolant with Extended Life Coolant (orange). If mixing occurs, drain engine cooling system and refill with originally equipped coolant type. If this contamination occurs the service change interval on Extended Life Coolant will be reduced from 6 years/150,000 miles to 3 years/30,000 miles.

1. Partially drain engine cooling system as described.
   

1. Disconnect engine control sensor wiring (12A581) from water temperature indicator sender unit (10884) .
   

1. Remove water temperature indicator sender unit.
   

Coolant Temperature Sender TESTING a coolant temperature sensor: Using an ohmmeter, measure the resistance between the terminals. Refer to the appropriate chart for your engine. If the resistance is not as specified, replace the sender. Fig. 1: Coolant temperature sender chart — gasoline engines
Fig. 2: Coolant temperature sensor chart — deisel engines
Fig. 3: Inspecting the coolant temperature sender for resistance

REMOVAL & INSTALLATION 1.8L and 1.9L Engines CAUTION
Never open, service or drain the radiator or cooling system when hot; serious burns can occur from the steam and hot coolant. Also, when draining engine coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze and could drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantities. Always drain coolant into a sealable container. Coolant should be reused unless it is contaminated or is several years old.

1. Locate the coolant temperature sending unit on the engine.
2. Disconnect the sending unit electrical harness.
3. Drain the engine coolant below the level of the switch.
4. Unfasten and remove the sending unit from the engine.
5. If the same sender is to be replaced, remove and discard the O-ring. To install:
   
6. If reusing the old sender, install a new O-ring.
7. Coat the new sending unit with Teflontape or electrically conductive sealer.
8. Install the sending unit and tighten to 18–22 ft. lbs. (25–29 Nm).
9. Attach the sending unit's electrical connector.
10. Fill the engine with coolant.
11. Start the engine, allow it to reach operating temperature and check for leaks.
12. Check for proper sending unit operation.

2.0L SOHC Engines

1. Disonnect the negative battery cable.
2. If equipped, raise the car and remove the left-hand splash shield bolts.
3. Loosen the petcock and partially drain the radiator.
4. Lower the car, locate the sender and unplug its electrical connection.
5. Unscrew the sender counterclockwise and remove it from the engine. Fig. 1: Unscrew the coolant temperature sender to remove it
   To install:
   
6. Install the sender and tighten it to 13–17 ft. lbs. (17–24 Nm).
7. Attach the sender electrical connection.
8. If equipped, raise the car and install the left-hand splash shield bolts.
9. Lower the car and fill the cooling system.
10. Connect the negative battery cable.

2.0L DOHC Engines

1. Disonnect the negative battery cable.
2. Loosen the petcock and partially drain the radiator.
3. Locate the sender and unplug its electrical connection.
4. Unscrew the sender counterclockwise and remove it from the engine. Fig. 2: On 2.0L DOHC (VIN 3) engines, tighten the coolant temperature sender to 89–124 inch lbs. (10–14 Nm)
   To install:
   
5. Install the sender and tighten it clockwise to 89–124 inch lbs. (10–14 Nm).
6. Attach the sender electrical connection and fill the cooling system.
7. Connect the negative battery cable.

Could be the coolant temp sensor:



4.0L engine shown





5.2L engine shown



To test it:

Perform this test on a cold or cool engine.

1. Disconnect the negative (-) battery cable.
   
2. Unplug the electrical wiring from the sending unit.
   
3. Using an ohmmeter, measure the resistance between the terminal and the sending unit's metal body
   
   1. Infinite resistance or zero resistance: the sending unit is bad, replace the sender with a new one.
      
   2. Other than infinite or zero resistance: continue test.
      
4. Remove the temperature sender from the engine.
   
5. Position the sending unit so the metal shaft (opposite end from the electrical connectors) is in a pot of water. Make sure that the electrical connector is not submerged and only the tip of the sending unit's body is in the water.
   
6. Heat the pot of water at a medium rate. While the water is warming, continue to measure the resistance of the terminal and the metal body of the sending unit:
   
   1. As the water warms up, the resistance goes down in a steady manner: the sending unit is good.
      
   2. As the water warms up, the resistance does not change or changes in erratic jumps: the sender is bad, replace it with a new one.
      
7. Install the good or new sending unit into the engine, then connect the negative battery cable.
   

1. With the engine cold, remove the ECT sensor.
   
2. Immerse the tip of the sensor in container of water.
   
3. Connect a digital ohmmeter to the two terminals of the sensor.
   
4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the sensor resistance illustration.
   
5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
   
6. If the sensor does not meet specification, it must be replaced.
   

Submerge the end of the temperature sensor in cold or hot water and check resistance



Temperature-to-resistance relationship of the ECT and MAT sensors

The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.
Fig. 1: View of the Engine Coolant Temperature (ECT) sensor Fig. 2: Engine Coolant Temperature (ECT) sensor locationTESTING
See Figures 3 and 4

1. Remove the ECT sensor from the vehicle.
2. Immerse the tip of the sensor in container of water.
3. Connect a digital ohmmeter to the two terminals of the sensor.
4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
6. If the sensor does not meet specification, it must be replaced.

Fig. 3: Intake Air Temperature (IAT) and Engine Coolant Temperature (ECT) sensor wiring diagram Fig. 4: ECT sensor temperature vs. resistance values