2001 Chevrolet/Geo Camaro  

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.

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

This diagnostic trouble code (DTC) will only run once per ignition cycle within enabling condition.

Conditions for Running the DTC
Conditions for Setting the DTC

The length of time for the engine coolant to reach a preset temperature is more than expected.

Action Taken When the DTC Sets
Conditions for Clearing the MIL/DTC
Diagnostic Aids
Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests for excessive resistance in the ECT circuit.

  2. 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?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important

The powertrain control module (PCM) will enable the engine cooling fans when certain engine coolant temperature (ECT) diagnostic trouble codes are set.

Is the cooling system low on coolant?

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Go to Loss of Coolant in Engine Cooling

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement .
  3. Connect a 3-amp fused jumper wire between the signal circuit and the low reference circuit of the ECT sensor.
  4. Turn ON the ignition, with the engine OFF.
  5. 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?

138C (280F)

Go to Step 7

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect the ECT sensor.
  3. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor and a known good ground.
  4. Turn ON the ignition, with the engine OFF.
  5. 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?

138C (280F)

Go to Step 5

Go to Step 6

5

  1. Test the ECT sensor reference low circuit for a high resistance or an open.
  2. Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 10

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6

  1. Test the ECT sensor signal circuit for a high resistance or an open.
  2. Repair the circuit as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 10

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7

  1. Remove the ECT sensor.
  2. Suspend the ECT sensor and a thermometer in a pan of water.
  3. Place the pan on a burner or hot plate.
  4. 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?

10-90C (50-194F)

Go to Step 8

Go to Step 9

8

Reinstall the ECT sensor.

Is the action complete?

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Go to Thermostat Diagnosis in Engine Cooling

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9

Replace the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement .

Did you complete the replacement?

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Go to Step 10

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10

  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

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Go to Step 11

Go to Step 2

11

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

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Go to Diagnostic Trouble Code (DTC) List

System OK