Code active, unplug coolant sensor dash gauge stops reading

Gauge is probably driven by computer and not the output from the temperature sensor and so gauge reading does not have a direct relationship with the actual engine temperature.

Generic obd2 scanner will read data/faults that can affect the exhaust emissions, which a computer driven gauge will not. Dealer/manufacturer level bi-directional software would be needed for in-depth inspections of all non-emission related systems.

Without testing ,hard to say for sure . Gauge could be going bad or the engine coolant temp sensor , wiring for sensor , PCM/ECM etc..... Knowing where that info originates an how it gets to the instrument cluster gauge is key in diagnosing the problem . Having worked at GM dealer ships an having correct tools an information gives me an edge over most . Most people would just replace parts an hope it fixes it . The first place to start would be to hook up a factory or professional scan tool an check for DTC'S - diagnostic trouble codes in the instrument cluster an the PCM/ECM . View live data inputs from the coolant temp sensor to the PCM?ECM - engine computer ,also see if that same info is getting to the instrument cluster . Most usually , if the problem is happening at the time we can't find the fault .
The Coolant temp sensor is an input to the PCM , the PCM sends this if to the instrument cluster on the serial data network . Don't know if you know what that is ?

Engine Coolant Temperature Gage
The instrument panel cluster (IPC) displays the engine coolant temperature as determined by the engine control module (ECM). The IPC receives a serial data message from the ECM indicating the engine coolant temperature. The engine coolant temperature gage defaults to 100°F (40°C) or below if:
• The ECM detects a malfunction in the engine coolant temperature sensor circuit.
• The IPC detects a loss of serial data communications with the BCM.
• The BCM detects a loss of serial data communications with the ECM.

GM did have problems with stepper motors inside instrument clusters 2003 to 2006 . Stepper motors are what control the gauge needles .
https://www.youtube.com/watch?v=oTm2oDx-_GM

No , their arn't two coolant temp. sensors . One near the thermostat housing . The Temp sensor is a input to the PCM / ECM . The PCM / ECM needs the temp info for fuel calculations . The temp gauge on the instrument cluster gets this info over class 2 serial data network Hooking up a factory or professional scan tool that can read sensor data is the easy way to diagnose ,not guessing . Looking at factory service info. an wiring diagrams too see what is what is another way . You have no clue ,take it to a qualified repair shop.

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while high temperature causes low resistance (70 ohms at 130°C/266°F).
The PCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the PCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.
The scan tool displays engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. A hard fault in the engine coolant sensor circuit should set DTC P0117 or DTC P0118; an intermittent fault should set a DTC P1114 or P1115. The DTC Diagnostic Aids also contains a chart to check for sensor resistance values relative to temperature.
The ECT sensor also contains another circuit which is used to operate the engine coolant temperature gauge located in the instrument panel.

What diognostics say its a sensor ?????? No diagnostic's ever tell's you it's a sensor . You as a technician would need to prove that by testing electrical circuits , viewing coolant temp sensor . Hooking scan tool and trying to command the gauge with by directional control . The temp gauge not showing any temp. ? No movement ? Engine Coolant Temperature Gage Inaccurate or InoperativeStep Action
Yes
No
Schematic Reference: Instrument Cluster Schematics
1
Did you perform the Instrument Cluster Diagnostic System Check?
Go to Step 2
Go to Diagnostic System Check - Instrument Cluster
2
Install a scan tool.
Turn the ignition ON, with the engine OFF.
With the scan tool, perform the Lamp and Gauge Check in the IPC Special Functions list.
Does the engine coolant temperature gage move up and down when commanded?
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3
3
Replace the IPC. Refer to Instrument Cluster Replacement .
Did you complete the replacement?
Go to Step 4
--
4
Operate the system in order to verify the repair.
Did you correct the condition?
System OK
Go to Step 2
Would hate for you to replace the coolant temp sensor an find out that's not the problem ! Does the vehicle start an run good ,trans mission shift good ? If so it's not the coolant temp sensor .
Engine Coolant Temperature Gage
The ECM sends coolant temperature as determined by the coolant temperature sensor to the BCM via CAN serial data. The IPC displays the engine coolant temperature with the gage when the BCM sends a class 2 message to the IPC indicating coolant temperature percentage. The engine coolant temperature gage defaults to cold (0°) or below if:
• The ECM detects a malfunction in the engine coolant temperature sensor circuit.
• The IPC detects a loss of class 2 communications with the BCM.
• The BCM detects a loss of CAN communications with the ECM.

The first hing I would do is look at the coolant temp sensor data on a scan tool. This will tell me what the computer is seeing. If it is reading something way different than it should I would unplug it and see what the value changes to. It should read -40F. if it reads anything other than there is a problem with wiring or the computer. If it does read -40 than I would likely replace the coolant temp sensor.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

to what? there are many
first post symptoms or job. why must be guess all that.
engine running bad, scan DTC errors? what?

some engines have up to 5 sensors. related to heat.
IAT, air temp
ECT water temp coolant
dash gauge heat.
mAF sensors have one. if opted.
a/C over heat sensors.

my guess is its the ECT.
am i correct?
this is part of the EFI fuel injection system.
click chapter fuel systems. chapter 14
page 50 , figure 40
see it, its screwed in to the thermostat housing
i bet it not bad.
the tests are supper easy, ask,
if water temp is 180f + hot.
and the scan tool shows, 180F , ITS NOT BAD.
easy huh?
http://www.jeep4x4center.com/knowledge-base/index.htm?utm_source=cj#service

ok , but no engine stated. which one>??
hot = lying dash gauge or real hand held pyro IR gun? a $20 tool.
did the scan tool ECT reading show too hot, Yes/no.

no new cap, really?
is the cam timed right?
is the spark timed right.?
is the ECU stuck in limphome mode? (huge heat to cylinder walls)
thermostat not upside down.
or thermostat not with bleed hole up hill trapping air below
making it lie and the dash gauge lie too, (the gauge can not measure air, period)

id not do squat with no IR gun. not me. its super accurate too.
and dirt cheap and saves burned hands,
and finds clogged rad tubes saving useless $250 new rad.
one tool. use it. and win.


i this this dang I6 engine, that loves to trap air?
had 3,now, over 500k miles on them. 1988 to present.
and never overheated but once, pin hole in heater core hose
ran bypass and drove home.
i have a 08 wrangler 6sp, 3.8L v6 , nice rig. easy to fix too.

there are 2 parts. (3 really)
engine make too much heat, (limphome or timed wrong)
air in system.
cooling system.

the rad pressure test pass and the green/blue test passes

no water in engine oil
no oil or carbon seen in the engine coolant , all places look.

how hot, ??????
200f
260F
280F"?
numbers matter, in all ofscience. they talk to you.
at what exact temp did the fans, kick in,?
the FSM shows you this, and how to do that. how to make sure the
fans kick in happen, RIGHT. some are 2 staged with A/C.
not just kick. (when)

old post and no symptoms, what reads wrong? or? scan fails or gauge? what?
for efi or the dash guage.? SAAB (2 "A"s?)
most newer cars is one sensor for both, FSM show this
as does alldata.com
ill answer for ECT.

2.0Liter. (google shows 500k hits but sure takes time to find it)
do you see the top rad hose, and it goes to the thermostat housing
and on said housing is the ECT.
this is for ECU usage. only. but ECU may repeat it to dash gauge.

Sounds like you replace the temperature sending unit(for the gauge on dash) what you need to replace is the Temperature sensor for the computer (ECM)

Since there is no voltage applied with the ignition off, if the gauge is not dropping down all the way, it is the gauge that has failed.
Temperature sensors are frequent failures but they are not active when the car isn't powered up. They can be intermittant, high, low but with no power, the gauge itself should go to the minimum.