Originally Posted by YelloEye

Have you tried the steps in post 7?
https://www.cobaltss.net/forums/prob...2/#post4283581

It's usually either a coolant temperature sensor or wiring.

Originally Posted by Bob_LT

Thanks YelloEye.
No, I'm kind of limited in my troubleshooting capability. I couldn't get the link to work either. I will try checking the wiring and maybe go ahead and replace the coolant sensor. The new thermostat seemed to help, just not a complete fix. It seems really odd the way this is engineered. But what do I know!

Originally Posted by rukkee

This might not help but this is GM's diagnostic instructions for that code. I am NOT a TECH

DTC P0128
Diagnostic Instructions
• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors
DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Diagnostic Fault Information
Circuit
Short to Ground
High Resistance
Open
Short to Voltage
Signal Performance

ECT Sensor Signal
P0117
P0118
P0118
P0118
P0116, P0128

Low Reference
--
P0118
P0118
P0118
P0128

Typical Scan Tool Data
ECT Sensor Temperature Circuit
Normal Range
Short to Ground
Open
Short to Voltage

ECT Sensor Signal
-39 to +120°C (-38 to +248°F)
150°C (302°F)
-40°C (-40°F)
-40°C (-40°F)

Low Reference
--
--
-40°C (-40°F)
-40°C (-40°F)

Circuit/System Description
The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and supplies a ground to the low reference circuit.

The purpose of this diagnostic is to analyze the performance of the thermostat, by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated , and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

Conditions for Running the DTC
• DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0502, P0503 are not set.

• The start-up IAT is between -7 to +55°C (+19 to +131°F).

• The start-up ECT is less than 50°C (122°F) when the start-up IAT is less than 10°C (50°F).

OR

• The start-up ECT is less than 75°C (167°F) when the start-up IAT is greater than 10°C (50°F).

• The engine run time is between 30 seconds and 30 minutes.

• The vehicle has traveled greater than 0.8 kilometers (0.5 miles) at greater than 8 km/h (5 mph).

• The mass air flow (MAF) average is greater than 1 g/s.

• This DTC runs once per ignition cycle within the enabling conditions.

Conditions for Setting the DTC
• The ECM detects that the minimum ECT of 55°C (131°F) has not been met with a start-up IAT of less than 10°C (50°F).

• The ECM detects that the minimum ECT of 80°C (176°F) has not been met with a start-up IAT of greater than 10°C (50°F).

Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.

Conditions for Clearing the MIL/DTC
DTC P0128 is a Type B DTC.

Diagnostic Aids
• DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.

• Inspect the ECT sensor terminals and the ECT harness connector for corrosion. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

• This diagnostic runs in a specific range. Measure and record the resistance of the ECT sensor at various ambient temperatures between -7 to +80°C (+19 to +176°F), then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

• A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

Reference Information
Schematic Reference
Engine Controls Schematics

Connector End View Reference

• Engine Controls Connector End Views

• Engine Control Module Connector End Views


Electrical Information Reference

• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

• Scan Tool Data List

• Scan Tool Output Controls

• Scan Tool Snapshot Procedure


Circuit/System Verification
Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

•Turn OFF the ignition.
•Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
•If the ignition has been OFF for 8 hours or greater , the ECT and the IAT should be within 15°C (27°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
•Use the scan tool to verify the proper operation of the engine cooling system fans.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.

•Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis .
Circuit/System Testing
Important: All electrical components and accessories must be turned OFF and allowed to power down.

•Ignition OFF, disconnect the ECT harness connector at the ECT sensor.
•Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal A and ground.
⇒ If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

•Ignition ON, verify the scan tool ECT Sensor parameter is less than -39°C (-38°F).
⇒ If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the ECM.

•Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT Sensor parameter is greater than 149°C (300°F).
⇒ If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

•If the circuits test normal, test the ECT sensor.
Component Testing
Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Engine Coolant Temperature Sensor Replacement

• Engine Coolant Thermostat Replacement

• Control Module References for ECM replacement, setup, and programming
© 2009 General Motors Corporation. All rights reserved.

Originally Posted by rukkee

This might not help but this is GM's diagnostic instructions for that code. I am NOT a TECH

DTC P0128
Diagnostic Instructions
• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors
DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Diagnostic Fault Information
Circuit
Short to Ground
High Resistance
Open
Short to Voltage
Signal Performance

ECT Sensor Signal
P0117
P0118
P0118
P0118
P0116, P0128

Low Reference
--
P0118
P0118
P0118
P0128

Typical Scan Tool Data
ECT Sensor Temperature Circuit
Normal Range
Short to Ground
Open
Short to Voltage

ECT Sensor Signal
-39 to +120°C (-38 to +248°F)
150°C (302°F)
-40°C (-40°F)
-40°C (-40°F)

Low Reference
--
--
-40°C (-40°F)
-40°C (-40°F)

Circuit/System Description
The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and supplies a ground to the low reference circuit.

The purpose of this diagnostic is to analyze the performance of the thermostat, by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated , and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

Conditions for Running the DTC
• DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0502, P0503 are not set.

• The start-up IAT is between -7 to +55°C (+19 to +131°F).

• The start-up ECT is less than 50°C (122°F) when the start-up IAT is less than 10°C (50°F).

OR

• The start-up ECT is less than 75°C (167°F) when the start-up IAT is greater than 10°C (50°F).

• The engine run time is between 30 seconds and 30 minutes.

• The vehicle has traveled greater than 0.8 kilometers (0.5 miles) at greater than 8 km/h (5 mph).

• The mass air flow (MAF) average is greater than 1 g/s.

• This DTC runs once per ignition cycle within the enabling conditions.

Conditions for Setting the DTC
• The ECM detects that the minimum ECT of 55°C (131°F) has not been met with a start-up IAT of less than 10°C (50°F).

• The ECM detects that the minimum ECT of 80°C (176°F) has not been met with a start-up IAT of greater than 10°C (50°F).

Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.

Conditions for Clearing the MIL/DTC
DTC P0128 is a Type B DTC.

Diagnostic Aids
• DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.

• Inspect the ECT sensor terminals and the ECT harness connector for corrosion. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

• This diagnostic runs in a specific range. Measure and record the resistance of the ECT sensor at various ambient temperatures between -7 to +80°C (+19 to +176°F), then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

• A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

Reference Information
Schematic Reference
Engine Controls Schematics

Connector End View Reference

• Engine Controls Connector End Views

• Engine Control Module Connector End Views


Electrical Information Reference

• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

• Scan Tool Data List

• Scan Tool Output Controls

• Scan Tool Snapshot Procedure


Circuit/System Verification
Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

•Turn OFF the ignition.
•Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
•If the ignition has been OFF for 8 hours or greater , the ECT and the IAT should be within 15°C (27°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
•Use the scan tool to verify the proper operation of the engine cooling system fans.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.

•Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis .
Circuit/System Testing
Important: All electrical components and accessories must be turned OFF and allowed to power down.

•Ignition OFF, disconnect the ECT harness connector at the ECT sensor.
•Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal A and ground.
⇒ If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

•Ignition ON, verify the scan tool ECT Sensor parameter is less than -39°C (-38°F).
⇒ If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the ECM.

•Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT Sensor parameter is greater than 149°C (300°F).
⇒ If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

•If the circuits test normal, test the ECT sensor.
Component Testing
Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Engine Coolant Temperature Sensor Replacement

• Engine Coolant Thermostat Replacement

• Control Module References for ECM replacement, setup, and programming
© 2009 General Motors Corporation. All rights reserved.

Originally Posted by Randall Flagg

Wiring. Check it.