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 ECT 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 DTC will only run once per ignition cycle within the enabling condition.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P0500, P0502, P0503, P1111, P1112, P1114, or P1115 are not present.
^ The coolant temperature is -40°C (-40°F) or more.
^ The coolant temperature is less than 70°C (158°F).
^ The intake air temperature (IAT) is -7°C (19°F) or more.
^ The engine has been running more than 2 minutes, and for less than 22 minutes.
^ The vehicle speed is more than 8 km/h (5 mph) for more than 2.5 km/h (1.5 miles).
^ The mass air flow (MAF) average reading more than 23 g/s.

CONDITIONS FOR SETTING THE DTC
The time for the engine coolant to reach a preset temperature has been exceeded.

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 is designed to detect a faulty thermostat.
^ Excessive coolant flow through the bypass hose to the throttle body may cause this DTC to set.
^ A skewed ECT sensor can cause a P0128 to set.
^ A skewed IAT sensor can cause this DTC to set.

TEST DESCRIPTION

Steps 1-7:

Steps 8-11:

The numbers below refer to the step numbers on the diagnostic table.
3. This step tests for excessive resistance in the ECT circuit.
4. This step tests for a skewed sensor through the range of temperatures affecting this DTC.