2005 GMC Envoy 4.2 L Failed emissions due to DTC P0128 (coolant thermostat- coolant temp below thermostat regulating temperature). Do I just replace the thermostat?

Which of these engines does the vehicle have ?
Engine Controls - 1.2L
Engine Controls - 1.4L DOHC
Engine Controls - 1.4L SOHC
Engine Controls - 1.5L
Engine Controls - 1.6L (L91)

Has to be - 1.6L

DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Circuit Description
The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed 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 the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, this DTC sets.

DTC P0441 Evaporative Emissions (EVAP) System Continuous Purge Flow
System Description
This DTC tests for continuous intake manifold vacuum flow to the Evaporative Emission (EVAP) System. The control module seals the EVAP system by commanding the EVAP canister purge solenoid valve Closed and the EVAP canister vent solenoid valve Closed. The control module monitors the fuel tank pressure (FTP) sensor to determine if a vacuum is being drawn on the EVAP system. If vacuum in the EVAP system is more than a predetermined value within a predetermined time, this DTC sets.

DTC P0700 Transmission Control Module (TCM) Requested MIL Illumination
This diagnostic trouble code (DTC) indicates that an emission related transmission DTC set in the transmission control module (TCM). The engine control module (ECM) receives the TCM DTC information over the serial data circuit. The ECM turns ON the malfunction indicator lamp (MIL) when the TCM sends a message over the serial data circuit requesting MIL illumination. The ECM will also store the freeze frame/failure records data for a transmission DTC. The DTC information for the ECM will only display DTC P0700, but the freeze frame/failure records data will display the transmission DTC that set.
DTC P1781
Conditions for Setting the DTC
• Transmission control module (TCM) detects that malfunction of engine speed signal for 2 seconds
• No DTC U2105
• Engine Misfire Detected

Does the speedometer work ?

DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulations Temperature
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.
The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected.
If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

P0481
Cooling Fan Relay 2 Control Circuit
PCM
DTC P0480 or P0481

Have you changed the thermostat ?
Do you know how to test cooling fan relay (S)
Battery positive voltage is supplied to the cooling fan 1 relay from the COOL FAN #1 fuse. The powertrain control module (PCM) controls the cooling fan 1 relay by grounding the low speed cooling fan relay control circuit via an internal solid state device called a driver.
Battery positive voltage is supplied to the cooling fan 2 relay and the cooling fan 3 relay from the COOL FAN #2 fuse. The PCM controls the relays by grounding the high speed cooling fan relay control circuit.
When the PCM is commanding a relay on, the voltage potential of the control circuit should be low, near 0 volts. When the PCM is commanding the control circuit to a relay, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the DTC will set.
The PCM will monitor the control circuit for the following conditions:
• A short to ground
• A short to voltage
• An open circuit
• An open relay coil
• An internally shorted or excessively low resistance relay coil
When the PCM detects any of the above conditions, the DTC will set and the affected driver will be disabled.

It's a thermostat/coolant issue. The definition is here:


P0128 Engine Coolant Temperature Below Thermostat Regulating Temperature



Possible Causes

• Check the operation of the thermostat (it may be stuck open)
• Coolant level is too low, or the coolant mixture is incorrect
• ECT sensor connector is damaged (check pins for damage, and for moisture)
• ECT sensor signal circuit has a high resistance condition
• ECT sensor has failed

P0128 is Coolant Thermostat ( Coolant Temp Below Thermostat Regulating Temperature ). The thermostat may be stuck open, preventing normal operating Temperature from beeing reached.How is the heat inside the car is it below normal ? Or the engine Coolant Temperature Sensor may be faulty and needs to be replaced. Good Luck and have a good XMASS.

DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Do you have any issues with heat in the vehicle? If so the thermostat may need replaced If not the engine coolant temperature sensor is most likely reading incorrectly due to a bad sensor or connection to the sensor.

P0128 - Coolant Thermostat (coolant Temp Below Thermostat Regulating Temperature). First check the coolant level,if its low then refill the coolant.If the coolant is checked full then get the thermostat replaced.-----------That's it.

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: 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

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember to rate and comment this).

DTC P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean?
This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms:
You will likely not notice any drivability problems other than the MIL illumination.

Causes:
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

Possible Solutions:
Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for a TSB such as this one. The ECM may need to be reprogrammed to correct the P0128 code


Hope helps (remember rated this).

P0128=Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature),
time to replace thermostat,coolant not at correct temp

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

P0410 - Secondary Air Injection System Malfunction
(has to do with exhaust system)

Note: There is a technical service bulletin (TSB) regarding code P0410 for a number of GM models. The TSB calls for the replacement of the AIR pump and the installation of a new secondary injection hose.