Error code P0128

The IAT sensor would not be a cause for this. I have only replaced thermostats to correct this concern, and I have done it on several engine variants with no reoccurring problems. Basically, the code sets in respect to how long the engine takes to reach operating twemperature, if I am not mistaken, and a poorly regulating thermostat would cause it. I suppose in theory, it could be the coolant temp sensor. If you have access to a scan tool that reads data, check and compare the IAT, and ECT after a cold soak to determine if there are congruent. Watching it heat up, the IAT is only going to get as warm as the incoming air, and the comparison is irrelevant at that point.

It is a 3.2, but is not a northstar, this engine is built by the folks that brought you the Opel. It is a bit of a challenge to do, and you are going to need more than just a six point socket set. This engine makes wide use of the external torx type of fastener, if you know what a torx fastener is, an external torx is a reverse model. You do not have to remove the timing belt, this is belt driven, not chain. You may not want to attempt if you have doubts, you could cause more problems than correct. The flat rate time for thermostat replacement is over 3 hours, for your information.

Is your a/c compressor on, and/or front defroster being requested? Either of these will override the fans on to pull air across the condenser, especially when the vehicle is not in motion. It may not be the case that the engine is never reaching operating temp, it could be that the thermostat is opening prematurely, adding time to the warm up process. If this wasn't a federal requirement, you may not even suspect you have an issue. The intention is to get the engine as warm as possible, as fast as possible, in an effort to lower the exhaust gas emissions, as fast as possible.

The system will still function as though it is coming on, although your compressor won't engage. As far as the cooling fans, I am not sure if they still come on or not. A faulty thermostat doesn't necessarily mean that the engine is not reaching operating temperature, it may lie in how long it takes to reach. I have included the literary portion of the diagnostic trouble code that may explain further.

CIRCUIT DESCRIPTION
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control, and as an enabling criteria for some diagnostics.

The air flow that is coming into the engine is accumulated. The accumulated air flow determines if the engine has been driven within conditions allowing the engine coolant to heat up to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach regulating temperature of the thermostat, diagnostics that use ECT as enabling criteria, may not run when expected.

If engine coolant fails to reach a present target temperature before a calculated air flow is accumulated DTC P0128 will set.

This DTC is continuously monitored during engine operation.

CONDITIONS FOR RUNNING THE DTC

DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, or P0118 are not set.
The engine block heater has not been detected or delayed for 20 seconds.
The engine off time is more than 15 minutes.
The engine speed is more than 960 RPM.
The engine coolant temperature (ECT) is less than 50°C (122°F) at start up.
The ambient air temperature is more than -39°C (-38°F) and less than 50°C (122°F).
The vehicle speed is more than 15 km/h (9 mph).
The mass air flow (MAF) average reading more than 23 g/s.
CONDITIONS FOR SETTING THE DTC
The time for the engine coolant to reach a set temperature has been exceeded.

ACTION TAKEN WHEN THE DTC SETS

The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
CONDITIONS FOR CLEARING THE MIL/DTC

The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
DIAGNOSTIC AIDS

DTC P0128 is designed to detect a faulty thermostat.
A skewed ECT sensor can cause a DTC P0128 to set.
A skewed IAT sensor can cause this DTC to set.

A code P0128 may mean that one or more of the following has happened:

Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor

Verify coolant strength & level
Verify proper cooling fan operation (check if it's running more than it should).
Verify proper engine coolant temperature (ECT) sensor operation,
Verify proper intake air temperature (IAT) sensor operation,
If the above items check out good, replace the thermostat

3.2L Engine

Drain the cooling system.
Remove the radiator inlet hose from the coolant outlet pipe.
Remove the bolt securing the oil level indicator tube, coolant outlet pipe and engine front lifting bracket from the cylinder head.
Remove the coolant outlet pipe from the thermostat housing.
Remove the O-ring seals from the coolant outlet pipe. Discard the O-rings.
Remove the intake manifold.
Remove the thermostat housing bolts.
Remove the thermostat housing with thermostat, from the engine.
Remove and discard the seal ring.
Remove the thermostat from the thermostat housing.
To install:

Install the thermostat into the thermostat housing.
Install a new seal ring.
Position the thermostat housing with thermostat, to the engine.
Install the thermostat housing bolts and tighten to 15 ft. Lbs. (20 Nm).
Install the intake manifold.
Lubricate 2 new O-ring seals with coolant.
Install the new O-ring seals around the coolant outlet pipe grooves.
Insert the coolant outlet pipe (3) into the thermostat housing.
Install the bolt securing the oil level indicator tube, coolant outlet pipe and engine front lifting bracket to the cylinder head and tighten to 15 ft. Lbs. (20 Nm).
Connect the radiator inlet hose to the coolant outlet pipe.
Fill the cooling system.

Intake manifold spacer. Torque the bolts in a spiral direction, starting from the inside and working outward to 15 ft. lbs. (20 Nm).
Intake manifold. Torque the bolts to 15 ft. lbs. (20 Nm).
Intake air resonator. Torque the nuts to 27 inch lbs. (3 Nm).(Note this is inch punds not foot punds)
Intake plenum. Torque the bolts to 71 inch lbs. (8 Nm).(note this is inch punds not foot punds)
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