How to diagnose stuck cold coolant temperature 5.0 L Ford

Has CHT - cylinder head temp. sensor . This sensor is a input to the PCM - engine computer . The PCM sends engine temp info on serial data network to the instrument cluster . I/C has a microprocessor inside . Have it check for DTC'S - diagnostic trouble codes . Not OBD2 codes , B - body codes . The gauge could be the problem , which means replacing the instrument cluster .
Engine coolant temperature information is received by the instrument cluster from the PCM over the SCP network. Temperatures over 121°C (275°F) will display hot (H). If the engine coolant temperature data is missing or invalid for five seconds, the instrument cluster will move the engine coolant temperature gauge below the cold (C) position.
The best way to diagnose this is with a scan tool , factory or a professional type . Know how the system works and not replacing parts is the proper way to diagnose .
U1073 D073 SCP (J1850) Invalid or Missing Data for Engine Coolant PCM CARRY OUT the PCM self-test.

• A single gauge does not return to zero with the ignition switch off

• Instrument cluster.

• INSTALL a new instrument cluster; REFER to Instrument Cluster . TEST the system for normal operation.

• Incorrect temperature gauge indication

• Circuitry.
• Instrument cluster.
• PCM.
• Engine coolant temperature sensor.

• GO to Pinpoint Test F .

I should also point out that if the temp sensor were bad the vehicle may not run right .
Using the factory scan tool
Trigger instrument cluster active command ENGCOOLNT. Monitor the engine coolant temperature gauge while adjusting the ENGCOOLNT active command to read 50% and 100%.

• Does the temperature gauge start at cold, move to half at 50%, and full hot at 100%?

Yes REFER to the Powertrain Control/Emissions Diagnosis (PC/ED) manual.

No INSTALL a new instrument cluster; REFER to Instrument Cluster . REPAIR the circuit. CLEAR the DTCs. REPEAT the self-test.
Your best bet , take it to a qualified repair shop !

Ensure thermostat is installed right side up or it wont allow proper flow. If thermostat is correct other problems could be Air in cooling system, low coolant level, plugged rad internal, water pump failure,coolant flow valve stuck,internal engine combustion leak to cooling system.
Auto Repair Diagnostics How to Diagnose Cooling System Problem

service the engine.. change oil ,filter plugs etc

did you clear the faults before each read
Diagnose Buick 3800 Engine
unusual code that

P0125 is the OBD-II generic code for the engine computer, or Engine Control Module (ECM), not detecting enough heat in the cooling system before the fuel management system can take control. The ECM has a predetermined value after you start your vehicle to expect a certain coolant temperature

P0125 OBD II Trouble Code Insufficient Coolant Temperature for Closed Loop...

ecm runs the engine cooling fans

3800V6 engine over heating and heater produces cold air
try these

EGR vale not working
overheating
inlet manifold gasket leaking
water pump fins corroded
soft radiator hoses
try a new radiator cap

internal heating
heater control tap stuck
directional actuator flaps stuck

You need to have a repair shop diagnose the problem

Your thermostat sound like it is stuck open.
How it works is your thermostat is designed to be closed when your engine is cold.
After you start your car your engine coolant circulates in the engine block.
This helps the engine reach its normal running temperature.
When your engine reaches the normal running temperature the thermostat slowly opens allowing the coolant to circulate through your radiator. the radiator cools the coolant and maintains your engine temperature so your engine does not over heat.
If your thermostat is stuck open your coolant is not permitted to allow the engine to reach normal running temperature. Causing your low temperature reading.
Hope this helps please give feedback good luck FixYa

Take a look at your temperature sensor and temperature sending unit, the former being the more likely of the two.

1- Check the coolant level. Is the reservoir showing full cold or hot level?
2- is the car at full operating temperature?

If the car is at full temp and the coolant is full you likely have a stuck/broken thermostat.

Let us know the above please.

possable problems:

1. coolant leak (level too low to fill heater core)
2. headgasket leak forcing the coolant out of the overflow.
3. stuck thermostat.
4. bad water pump. if the impelers are broken, coolant wont flow.
5. HVAC malfunction. system may think you selected cold or is unable to move diverter. maybe a fuse,solenoid or vaccum leak? (hvac controler uses vaccume controled soleniods)
6. airlock. if you have resently changed your coolant, you may need to repeatedly squeeze the heater core hose to help force out the air.

The larger of the two is the coolant temperature sensor. The other is the actual gauge temperature sensor. The coolant temp. sensor gives information to the cars computor & when the car reaches a predetermined temperature, the computor changes the ignition timing allowing the car to run at normal operation temperature settings. The temperature sensor gives info to the dash board gauge &/or temp. warning light. The reason you can't start on cold days is that your coolant temp. sensor is stuck in advanced mode & the timing is to far advanced . To remedy this problem change the coolant sensor. Good luck!

Trouble Code: P0128

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Possible Causes:


Insufficient warm up time Low engine coolant level Leaking or stuck open thermostat Inoperative engine coolant temperature (ECT) sensor Inoperative Cylinder Head Temperature (CHT) sensor