Here you go guys! Long answer from Techline. Long story short, it does sound like the gen needs to be removed to get to the darn thing. R2 instructions are in the 2nd document.
Good Luck!
Dave
Document ID# 735601
2002 GMC Truck Envoy - 4WD
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DTC P0128
Circuit Description
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 engine has been driven within 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 regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not run when expected.
This diagnostic trouble code (DTC) will only run once per ignition cycle within the enabling conditions.
If the engine coolant fails to reach a preset target temperature before a calculated air flow is accumulated, DTC P0128 will set.
Conditions for Running the DTC
DTCs P0105, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0130, P0131, P0132, P0171, P0172, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0325, P0336, P0420, P0440, P0442, P0446, P0452, P0453, P0480, P0502, P0503, for automatic transmission only, and P1441 are not set.
The engine has run more than 30 seconds and less than 20 minutes.
The minimum intake air temperature (IAT) is more than -7°C (19°F).
The engine coolant temperature at start up is less than 65°C (149°F).
Conditions for Setting the DTC
The average air flow is more than 20 g/s.
The vehicle has traveled more than 2.4 km (1.5 mi) over 40 km/h (25 mph).
Enough air flow has entered the engine and the engine coolant temperature has not risen to 80°C (176°F) for 30 seconds.
Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) if a failure is detected during 2 consecutive key cycles.
The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame/Failure Records.
Conditions for Clearing the MIL or DTC
The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.
A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
Use a scan tool in order to clear the DTCs.
Diagnostic Aids
A DTC P0128 is designed to detect a faulty thermostat.
A skewed IAT sensor may cause this DTC to set.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
This step tests for excessive resistance in the ECT circuit.
This step tests for a skewed sensor through the range of temperatures affecting this DTC.
Step
Action
Values
Yes
No
Schematic Reference: Engine Controls Schematics
1
Did you perform the Diagnostic System Check-Engine Controls?
--
Go to Step 2
Go to Diagnostic System Check - Engine Controls
2
Is the cooling system low on coolant?
--
Go to Loss of Coolant in Engine Cooling
Go to Step 3
3
Turn OFF the ignition.
Disconnect the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement .
Connect a 3-amp fused jumper wire between the signal circuit and the low reference circuit of the ECT sensor. Refer to Using Fused Jumper Wires in Wiring Systems.
Turn ON the ignition, with the engine OFF.
With a scan tool, observe the ECT sensor temperature.
Does the scan tool indicate that the ECT sensor temperature is more than the specified value?
138°C (280°F)
Go to Step 7
Go to Step 4
4
Turn OFF the ignition.
Disconnect the ECT sensor.
Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor and a known good ground.
Turn ON the ignition, with the engine OFF.
With a scan tool, observe the ECT sensor temperature.
Does the scan tool indicate that the ECT sensor temperature is more than the specified value?
138°C (280°F)
Go to Step 5
Go to Step 6
5
Test the ECT sensor low reference circuit for a high resistance or an open.
Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 10
--
6
Test the ECT sensor signal circuit for a high resistance or for an open.
Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 10
--
7
Remove the ECT sensor.
Suspend the ECT sensor and a thermometer in a pan of water.
Place the pan on a burner or hot plate.
Test the resistance of the ECT sensor through the specified range of temperatures.
Does the resistance and temperature closely match the Temperature vs Resistance table?
10-90°C (50-194°F)
Go to Step 8
Go to Step 9
8
Install the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement .
Is the action complete?
--
Go to Engine Fails To Reach Normal Operating Temperature in Engine Cooling
--
9
Replace the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement .
Did you complete the replacement?
--
Go to Step 10
--
10
Use the scan tool in order to clear the DTCs.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC run and pass?
--
Go to Step 11
Go to Diagnostic Aids
11
With a scan tool, observe the stored information, Capture Info.
Does the scan tool display any DTCs that you have not diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List
System OK
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Document ID# 735601
2002 GMC Truck Envoy - 4WD
Document ID# 735499
2002 GMC Truck Envoy - 4WD
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Engine Coolant Temperature (ECT) Sensor Replacement
Removal Procedure
Notice
Use care when handling the coolant sensor. Damage to the coolant sensor will affect the operation of the fuel control system.
Turn the engine OFF.
Caution
Before servicing any electrical component, the ignition key must be in the OFF or LOCK position and all electrical loads must be OFF, unless instructed otherwise in these procedures. If a tool or equipment could easily come in contact with a live exposed electrical terminal, also disconnect the negative battery cable. Failure to follow these precautions may cause personal injury and/or damage to the vehicle or its components.
Disconnect the negative battery terminal.
Drain coolant below the level of the ECT sensor. Refer to Draining and Filling Cooling System in Engine Cooling.
Disconnect the ECT sensor electrical connector (1).
Remove the drive belt and the generator. Refer to Generator Replacement in Engine Electrical.
Carefully remove the ECT sensor (1).
Installation Procedure
Notice
Use care when handling the coolant sensor. Damage to the coolant sensor will affect the operation of the fuel control system.
Notice
Replacement components must be the correct part number for the application. Components requiring the use of the thread locking compound, lubricants, corrosion inhibitors, or sealants are identified in the service procedure. Some replacement components may come with these coatings already applied. Do not use these coatings on components unless specified. These coatings can affect the final torque, which may affect the operation of the component. Use the correct torque specification when installing components in order to avoid damage.
If installing the original sensor or a new sensor without sealant, apply thread sealer P/N 12346004 or equivalent.
Notice
Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Install the ECT sensor. Tighten
Tighten the ECT sensor to 16 N·m (12 lb ft).
Connect the ECT electrical connector (1).
Install the generator and the drive belt. Refer to Generator Replacement in Engine Electrical.
Connect the negative battery terminal.
Refill the engine coolant. Refer to Draining and Filling Cooling System in Engine Cooling.
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Document ID# 735499
2002 GMC Truck Envoy - 4WD