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

DTCs P0112, P0113, P0117, P0118, P0125, P0500, P0502, P0503, P1111, P1112, P1114, or P1115 are not set.
The coolant temperature is more than -40°C (-40°F).
The air temperature is more than -7°C (+19°F).
The engine is running more than 4 minutes.
The vehicle speed sensor (VSS) average is more than 24 km/h (15 mph) over the key cycle.
The MAF average reading is more than 23 gm/sec.
The vehicle has been driven more than 4.8 km (3 miles).

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

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 will set if the diagnostic does not pass within 25 minutes after start up.
DTC P0128 is designed to detect a faulty thermostat.
A skewed ECT sensor to can cause DTC P0128 to set.
A skewed IAT sensor may cause this DTC to set, the IAT sensor should read near ambient temperature.

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	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?	--	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. 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 reference low 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 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	Reinstall the ECT sensor. Is the action complete?	--	Go to Thermostat Diagnosis 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 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?	--	Go to Diagnostic Trouble Code (DTC) List	System OK