Code P0128

Good afternoon,

It could be the sensor but I doubt it. It is about the temperature of the coolant is too low.

https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works

What temperature thermostat did you replace the original? You need a 195-degree thermostat.

https://www.2carpros.com/articles/replace-thermostat

I would try a new thermostat. That is the most common failure.

Roy

Removal and Installation

1. With the vehicle in NEUTRAL, position it on a hoist. For additional information, refer to Vehicle Jacking and Lifting. See: Vehicle Lifting > Procedures > Jacking and Lifting.

2. Drain the cooling system. For additional information, refer to Cooling System Draining, Filling and Bleeding See: Cooling System > Procedures > Cooling System Draining, Filling and Bleeding.

3. Disconnect the positive battery cable. For additional information, refer to Section 414-01 See: Battery > Removal and Replacement > Battery.

4. Disconnect the lower radiator hose from the thermostat housing.

5. Remove the 2 bolts, thermostat housing cover, O-ring seal and thermostat.
To install, tighten to 10 Nm (89 lb-in).

6. NOTICE: Use Motorcraft(R) Specialty Green Engine Coolant or Motorcraft(R) Specialty Orange Engine Coolant. Use the same type of coolant that was originally used to fill the cooling system. Do not mix coolant types. Mixing coolant types degrades the coolant corrosion protection and may damage the engine or cooling system.
NOTICE: Motorcraft(R) Specialty Green Engine Coolant is very sensitive to light. Do NOT allow this product to be exposed to ANY LIGHT for more than a day or two. Extended light exposure causes this product to degrade.

To install, reverse the removal procedure.

Clean and inspect the O-ring seal. Install a new seal if necessary.

Lubricate the thermostat O-ring seal with clean engine coolant.

7. Fill and bleed the cooling system. For additional information, refer to Cooling System Draining, Filling and Bleeding See: Cooling System > Procedures > Cooling System Draining, Filling and Bleeding.

Check and test the engine coolant temperature sensor-

Best is you can supply us with the codes so we can help in pin pointing the troubled area
Coolant below thermostat temp is an indication of a bad thermostat (open)and need to be replaced

P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) Detailed Description
Indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.

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
WE FIND IT VERY COMMON FOR THE THERMOSTAT TO BE AT FAULT
CHECK COOLANT 1ST